Write to support@groovelabapp.com.
Problems
Include what you were doing and what the app did instead, your device and iOS version, and the link to the sequence if it happens on one in particular. A screen recording is worth sending for anything you can hear rather than see.
Restoring a purchase
Groove Lab is a single purchase covering every device signed in to the same Apple ID, so reinstalling it is free. Open the App Store listing and tap Restore. Write in before paying a second time.
Feature requests
Say what you were trying to practice and what got in the way.