My pixie has been doing the same thing and I was worried what was contributing to this behavior. Since I haven't had him more than a year I can't figure out if it's just seasonal behavior or something I was doing wrong. I had him on all natural top-soil that I used about a month and a half ago right about the same time this behavior started. The soil had sticks, and other miscellaneous pieces of forest debris in it. After he would bury for a week or two, I knew it didn't seem right so I would put him in a tub and soak him in warm water. He went poop and then repeated the same behavior burrowing back into the dirt and acting real lethargic barely eating. Then after a couple other weeks I soaked him in warm water again, but this time also added about a tablespoon of honey into the water. I tried this thinking that the honey would making him snap out of his lethargic behavior, but it didn't change much.

Then, I thought maybe he was irritated from the dirt he was in. So I changed the dirt back to coconut fiber yesterday and fed him last night in it in which he actually did eat and today he decided to start soaking in his water source again . Ok so back to your issue, sorry I just wanted to share this as I felt it was so closely related to what you're experiencing. If your pixie is still a juvenille under 4-5 inches, I wouldn't let him or her burrow for more than 4-6 days. Semi adult-adult 4.5-8+ inches I wouldn't let burrow for more than two weeks without eating depending on the size (just isn't normal behavior), unless you are putting them through aestivation (hibernation) which is a whole other topic we won't get into. I don't use the Celsius measurement, but it seems like you know what you are doing as far as temperature/humidity conditions and diet.

The only way to pinpoint your issue would be start with the following: Try soaking in warm water with honey for 30 minutes or more. Does dirt dry too fast? Is the water source easy for him or her to get into and out? Where is the heat source? Reduce dirt level to try to reduce burrowing behavior. I'm guessing some pixies don't handle human contact as well as others and feel the need to hide by burrowing under the dirt to feel safe from predators (you; not trying to be mean, but imagine a huge object approaching you). The bigger pixies typically handle human contact or exposure better than young ones since they have grown accustom to that daily lifestyle.