Copy-paste error?

I was working on a song, and I copy-pasted the chorus onto the end of the last verse. On the new chorus, the drumset part's notes disappeared, and got replaced by blank measures, with the rests gone. They were just... blank. No notes, and no 4-beat rest. Musescore wouldn't let me insert notes onto the blank measures. The note head appears underneath my cursor, but it disappears when the cursor gets to the blank measures. I tried selecting the entire score and copy-pasting it into a new, blank score, but the blank measures are still there.

Does anyone know what is going on?