The code for building the migration graph wasn't considering

migration-aware apps that didn't already exist in the database. This

could prevent some migrations from applying or being considered for

custom evolutions.
The cause was a small logic error where we tried to avoid working with

the signature if it didn't exist, but we checked this too early and

failed to put the app in the list of migration-aware apps. The logic has

moved to the appropriate conditional.

All unit tests passed.
Verified that this fixed an issue with an newly-added app not being

considered for migration.