HCA: Hermit Crab Association

My hermit crab has buried himself on 21/4 and came out today. We thought he was molting because he dug hole a lot before burying, but it seems he didnt molt. His skin color is same as before.

Any reason to explain why he buried himself for nearly a month?

My another hermit crab buried himself into soil today in another tank, but he came out after few hours. He had a few symptoms of molting including eating a lot, drinking a lot of water, staying near the water and decrease in activity level. Is he preparing for molting?

They are both strawberry hermit crab and I use sand and coconut soil. Temperature is around 22-28degree and humidity is between 85-90% of both tanks.

Crabs will dig to de-stress. Or sometimes just for fun.
Is your substrate deep enough and moist enough?

The substrate is 3 times the height of my hermit crab and we sprayed water every day. So, it should be moist enough.

Does hermit crab like living alone or in groups? Because the 2 hermit crabs are put in the same tank now and one of them starting digging soil. Is it for fun or he doesn't like the environment?

They live in groups in the wild, and most of us recommend having at least two. They like having friends. Digging is normal for them, if they are under the substrate, don't dig them up.
How are your heat and humidity?