The one I use can populate the clipboard with usernames, passwords, etc. for easy pasting but I'm pretty sure it clears the clipboard after a time for security.
If the clipboard is used for something else within that time window, that could be cleared out instead, I imagine. Caveats being that maybe mine is smarter than that or my memory isn't great so even if it has happened I don't remember. The fact this idea occurred to me makes me wonder though.
Also, different applications do things differently so maybe even if mine doesn't, yours might, etc.
The fact it was a one-off might also tie in because we log into things about as sporadically as one-off weird things happening, and you might not have connected the two, so to speak, if that's what's happened.