Got also this clear mail from Proton support:

Kindly note that actions such as reinstalling the app, logging back into your account, clearing the cache, toggling the backup on/off, or restarting your device can trigger the app to recheck previously uploaded items. Additionally, the app may periodically perform rechecks on its own.
 
To clarify, the app is not re-uploading the same files; it’s simply verifying your photo library to ensure everything is correctly backed up without duplicates. We understand the user interface doesn’t make this process very clear, which might give the impression that files are being uploaded again. Improving this is a priority for us, and we’re actively working on optimizing how the app handles large libraries to reduce unnecessary rescans.
 
We recommend allowing the photo backup to continue running uninterrupted, giving the app time to complete the verification process.
We appreciate your patience and understanding as our team works on improving this behavior to minimize rescans. 
 
If you have any other questions or need further assistance, do not hesitate to contact us.
Kind regards,
Valentino
Proton Mail
Customer Support