Hi.
Polypterus dig for two reasons.
1. As a way of hunting live prey.
2. As a way of hiding from predators, or being stressed.
It is most likely the reason number 2 that is affecting you.
Find out what is stressing it out.
*edit*
Just looked at your pics, and can garuantee you it's the Pleco that is stressing it.
Plecos (especially the one you have in your tank) are the number one NO NO with Bichirs.
They can and DO suck the slime coat of Bichirs, especially in the dark.
Your Bichirs will eventually die if you insist on keeping a pleco such as you have with them.
Also, you are keeping your Bichir on gravel that is deadly for Bichirs.
They are messy eaters, and are known to take in substrate with food.
Gravel like you have will cause compaction, which leads to a very nasty death.
I can tell you right now, it's not the plecos, I've had Bob for about a year now, and the common pleco was just adopted a few weeks ago. Originally he was supposed to go in my planted tank, but I decided against it.
The other pleco was given to me from my ex about about a month ago, after I told him his tank is too small for it.
Bob starting burrowing, or at least trying to about 9 months ago, before I got my plecos. I'm actually up for most the night and since the tanks in my room I'm constantly watching it and I've never seen either of them suck on him. However about 4 months ago I had to get rid of my sailfin because he was sucking on him, I had him for about a month as soon as I caught him doing it I gave him away. But I will keep a closer eye on it..
About the gravel, I'll be changing that soon, It's only in a small section of the tank, the only reason I put it in there was for the beneficial bacteria. When I got the larger tank I put it in the place of the other one, so it ended up being a quick switch. I didn't cycle it because I had nowhere to put my fish while I did so. But my bichirs and rope fish are usually hand fed anyway, I've never seen any of them eat gravel.