Every time I drop the rifle and it falls into a den, it's not thrown back as other items used to. It just disappears. It also happens if a lizard steals the rifle from my hands and drops it in a den.
It's pretty annoying because I use this perk as Saint and I can't eat blue fruits if I don't drop the rifle, so I have to be careful where I drop it every time I want to eat. I only saw it happening as Saint, but I suppose it happens with other slugcats as well. Also, the video records and my previous experience makes me think it only happens with dens that a creature spawned in the cycle, not any dens.