That happened to me. I gave Veronica a ballistic fist and several hours of play later I was rifling through her inventory and I noticed it had disappeared. I'm not 100% sure but I think companions discard their weapons once they've fully degraded and revert to their default.
The weapon probably broke. Followers drop a broken weapon the moment it breaks (and followers get their weapons damaged a lot by stray bullets). I've seen Arcade drop a Gauss rifle I gave him, and it was broken.