I had Cass using the Dinner Bell unique shotgun. I just realized she reverted back to her standard weapon. It was in full repair when I gave it to her and she has more than enough ammo. I don't think she's used it enough for it to break, and I'm fairly certain it didn't get shot out of her hands. Aside from which, it's missing from any save I load, even saves where I know for a fact she had it. What gives?

I've read here that there are some bugs when entering/leaving casinos with companions that they can sometimes drop weapons. I can't confirm this, as I leave my companions outside the strip - but idk.

I had it happen to me once, but with a Thermite Lance. It is not on the ground anywhere. But it was there when I reloaded, so it is probably not the same bug as yours - or that you didn't notice for a long time that it was gone