I actually fixed it when I found a backup that works, but then it did it again just now. We're supposed to play Sunday, so I'm on another backup.
It looks as though it happens if I leave characters in the initiative tracker after a session. Not sure if it's player characters or NPCs, but I'm leaning toward PCs.
Here's the current broken file. I'm going to manually rebuild because we're supposed to play again Sunday, but I'll check back just in case. If not, no worries, but any information on the cause so I can prevent it in the future would be awesome.
Thank you, Trenloe.
https://drive.google.com/file/d/1P6R...ew?usp=sharing