Once zeitwerk mode is enabled and the configuration of eager load paths double-checked, please run: bin/rails zeitwerk:check
This is helpful because it allows you to detect/find eager load issues in dev before/without deploying.
I ran into an eager load error after deploying to staging...
Final Form makes the assumption that your validation functions are "pure" or "idempotent", i.e. will always return the same result when given the same values. This is why it doesn't run the synchronous validation again (just to double check) before allowing the submission: because it's already stored the results of the last time it ran it.