Sorry Amos, I should've been clearer - after reinstalling the game, it found the manifest fine. Beforehand it didn't. My suspicion is that renaming was the cause of the error, but I haven't verified that. It would mean the repro would hypothetically be:
- Create a game with a manifest
- Install the game, note it finds the manifest fine
- Rename the game
- Publish an update to the game
- Update the game in the itch app
- Launch the game - it doesn't find the manifest
- Uninstall and reinstall the game - it finds the manifest
This is basically more or less what I did, I think. But there were other factors involved probably, such as updating the itch app itself somewhere in the middle there.