Is Evolver bugged?

[RedSage]

So I was lucky enough to not only get it in my current run, but also have Monster Blood as well, so it synergizes to evolve every 3 unique enemy types killed.

However, the gun keeps devolving. If I switch off the gun, and defeat enemies with other guns and come back to it, it's usually a stage or two down from where it was. Even more appallingly is that I JUST had it in the dragon form, cleared the room with it, and then it turned back into the sponge.

Is the wiki missing info? Or is this indeed a bug?

[MoonBeech]

I believe the Evolver devolves naturally if you leave it alone or equip other guns instead of it for a while. That sounds like it devolved pretty fast, but maybe as it evolves into later iterations it also takes less time for it to devolve?