Did you set --max-per-region by chance? If your app has three regions and you run fly scale count 3 --max-per-region=1, then you remove a region – it might hang forever waiting for something it can’t do (try to place 3 vms in only two regions).
The errors in the initial post were very specific to builders, if you’re seeing no output at all it’s likely something else is wrong.
If your app reboots randomly, you can troubleshoot with fly status --all and fly vm status <id>. That will show you if it’s exiting with a weird code, or if health checks are failing. Both can cause a restart.