I've created an app which saves data to Proton Drive folder. The issue happened was:
- When repeatingly save to a file, Proton Drive fails to sync, eg. press Ctrl+S continuously
- When saving multiple files at once, happened to fail too

I guess the issue is when a file is changed, Proton uploads immediately, but instantly while Proton Drive is uploading, the file is changed, and it conflicts.

Other drives such as MEGA, Google Drive, seem to upload only after a few seconds when the file is idle.