Sorry for the snag!
Sounds like a weird one, let's try to find what's causing it:
Are you seeing the same issue if you create a new project?
Could you please check on the version you are running? We just released version 3.3.1 which is available here if needed: https://balsamiq.com/download
If the issue persists, would you mind sending us your BMPR file to email@example.com so that we can try to reproduce the issue here? We will keep your data in the strictest confidence and only long enough to try to debug your issue.
Sorry again for the hassle with this, we'll get to the bottom of it!