A shot in the dark: Consider switching to a brand new Fly Builder to see if that alleviates the problem?
# grab the name of the builder
flyctl apps list | grep 'fly-builder' | cut -f1
# destroy (exercise caution!)
flyctl apps destroy <builder-name>
The last few weeks, about 50% of deploys fail to complete
May be: Creating release error when trying to deploy rails app for the first time - #2 by ignoramous?