Safari 13.1+ on macOS, as well as Safari on iOS/iPadOS 13.4+, introduced a seven-day cap on all locally-stored data by websites. This means that unless you open PhotoStack at least once a week, your saved watermarks could be deleted.
The only way to avoid losing your watermarks is to export them as files (from the watermark list), then re-import them after Safari wipes PhotoStack's data.
In addition, PhotoStack may not be able to download files (completed images, exported watermarks, etc.) when it is opened from the iOS home screen. To get around this, open edit.photostack.app in your browser instead.