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, then re-import them after Safari wipes PhotoStack's data. To export a watermark, select it in the dropdown menu, press the Watermark button at the bottomtop-right of the screen, and select the export option. The import function can be found on the same button.
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.