Safari has a seven-day cap on all locally-stored data by websites in version 13.1 and higher. Unless you open PhotoStack at least once a week, your saved watermarks could be deleted.

You can avoid data deletion, and hide the warning in PhotoStack, by adding PhotoStack to your Home Screen on iOS or Dock on macOS. The installed version of PhotoStack uses its own data storage that is not automatically deleted.

You can also periodically export your watermarks as files, then re-import them after Safari wipes PhotoStack's data.