iCloud Backup and Apple Watch apps

When I upgraded my iPhone I went through a mostly-automated process pairing my Apple Watch to the new iPhone. It includes the watch backing up, erasing itself and then restoring from the backup. My app Pinbook stores files only on Apple Watch. After the restore process had finished, I opened Pinbook and realised that all my files were gone.

Apple support article:

Here’s what your Apple Watch backup includes

App-specific data (for built-in apps) and settings (for built-in and third-party apps).

In conclusion, Apple Watch iCloud Backup does not include app-specific data for third-party apps. For Pinbook, I need to figure out a way to backup data to iPhone or, better, use CloudKit sync.