I think this falls into the not-enough-info category. As usual, it is always best to investigate possible medical issues first, as you have done, but it sounds like this doesn't happen when your husband is home so it could be behavioral. I should note that my professional experience is not in dog behavior, but my human behavioral experience does have some comparable behaviors regarding "reward response." In general, if it isn't due to some medical reason, which I think you are wise to determine first, there may be some reason your little guy/girl is responding to your husband in different manner than he/she responds to you or others. The most obvious, but not necessarily correct response is that the dog respects your husband more, which (IMHO) is easy to correct. If your dog sees your husband as "large and in charge" he/she may not feel as confident when left in your care. If that is the only issue there are ways to make yourself be seen as the gentle but assertive leader that helps most dogs feel comfortable. Of course, that is only one opinion.
Also, you don't mention how long you have had this pup and if he/she was rescued which would account for all kinds of behaviors that will pass with time, patience, love and consistent assertive, but very encouraging, confidence building treatment. I learned, with the help of a trainer, one of the best tricks for building confidence was teaching Sully to raise her ears rather than holding them back low, a sign of dominance. The only way I could teach her was to wait until her ears were up then praise her and give her a treat. It really seemed to help. Now if she approaches with he less confident behavior I can say "Show me your ears" and give her praise and a treat when she does-very cute by the way. I also ignored her showing her belly, also a sign of dominance, by turning my back on her when she went belly up and praising her when she sat up. Again, this may not help you, but it is worth considering whether your little dog is acting less confident in your presence and that is leading to the hiding. If she/he was a rescue she may be hiding because she or he is afraid to be taken away again. I was the first person my dog saw when she arrived from a long trip several states away. She was terrified but she learned to trust me fairly soon. It took much longer for her to trust my family and she still hides from neighbors that she normally loves, if they come by when I am not home, which happened a few of times because I gave a key to the neighbors in case of an emergency. When I am home Sully runs to any neighbor, but apparently she panics and runs away when I am not home. She won't even allow them to take her out if I get sick and I am home. Somehow she senses that family is different and she will go anywhere with any of them, but they are all people with lots of pet experience so she may just be feeling confident with them. Hard to say. I am sure you troubles will pass, but it is hard to say for sure without more info. I am sure others on this site with more extensive animal behavior experience will offer some great advice you can use! Good luck!!
Sorry, I meant to say that presenting her or his belly was a sign of submission, not dominance. In my opinion it is the dog's attempt to say "You are the boss. Please don't hurt me." I would much rather she say "You are the boss. I love you and I want to listen to you." I see that much more often now and it is so good to see a dog that feels like a partner rather than as my boss or maybe worse, as a pitiful slave to my whims. I hope you see a quick resolution to your problem.
I laughed when you said that presenting the belly is a sign of submission. When Bella does that to me , It generally means "No, I'm not moving and this is the most awkward posture I can assume so that you can't lift me easily!". I can actually observe the machinations in her considerable corgi brain, under "operating instructions for inconvenient owners". Gently drop right shoulder to ground, while maintaining eye contact with your human. Keep smiling at them so that they know that it is not personal, and in no way diminishes from the quality of the relationship. It is essential to keep the human in side, as the provider of treats & walks, but do not allow them to be under the misapprehension that they are actually in charge, and if it does not suit you to adhere to their wishes, they are fairly powerless when confronted with a smooth canine belly, four cute paws and a clear "no hard feelings, but I'm not moving expression". Just to keep them nicely confused, sporadically carry out their instructions: they get a great kick out of it, and it can trigger the treat delivery response.
Reading the original issue, it sounds to me like corgi psychology: Bella sulks and takes to her crate if she is not getting enough stimulation from walks. As much as the physical exercise, she really needs the mental exercise. Dougal, by contrast, would happily exercise from kitchen to living room, a slow saunter to bark at the neighbour's cats, and then take the weight of his feet. Interestingly, while very submissive in nature, ne does not roll on his back.
Agree with Holly, not enough information to even make an educated guess.... something in the house may be making him nervous. has anything changed with you recently (health, mood, habits?) Try thinking back to when you first noticed something was off and you may get some insight. You can't always figure out why a dog is doing something, but you can usually do something about it. Make his time with you special, and have something yummy waiting for him at home when you return from your walks. Having him checked by the Vet is always a good idea to eliminate any physical concern.
He may have a preference to your husband. Do you or your husband spend more time with your dog? We had similar situation before. I work in an office and my boyfriend works from home. Obviously he spends way more time with our dog than I do. So in the beginning our dog would listen to him more and behavior better when he was around. Then one day I decided that I am his owner also so I get up early to feed him, and train him. I also take mine for a walk after I get back from work. I am assertive and dominating (to my dog obviously) so my dog listens to me even though he spends more time with my boyfriend. He may feel bored, lonely or scared. Make sure to spend time play with him or train him (which is my preference since the only game my dog likes to play is chasing people) when it's just you and your dog at home.