I can’t give any specifics (someone from Fly will be able to) however the general cause is likely the recent issues with the Seattle (sea) region, not anything you’ve done:
Fly did hardware migrations there …
… and so I suspect your instances (yes, that is the correct word, or you could also use ‘vm’, short for virtual machine, which I’ve seen Fly tends to use instead, such as for billing) were migrated from there to other regions. In order to get the app running again.
If at that time your regions list did have other regions/backups, Fly would have probably migrated the vm to one of them. As generally it’s better to have an app running in a different region than not running in the same region. Else if a region is down for any reason, your app would be down too until that’s resolved. Which would likely take longer than simply moving a vm.
If you only want your app to run in sea, I believe you can “pin” it to a region by attaching a volume to it in that same region. That used to be possible anyway:
I’m not sure what happens if sea has any future issues and your regions list only includes sea though.