I don't think syncing Joplin with your desktop folder really counts as syncing with Proton Drive. You are syncing locally, then Proton Drive is taking over and syncing your local folder with the cloud. You have a two step process. For Joplin to sync with Proton Drive (like Dropbox or NoneDrive) it would need an API to connect to, and Proton doesn't have that. As Laurent said, this would require a proxy like ProtonBridge for Mail. This two step process may seem to be functional (I too tried it and lost a ton of notes) but will eventually cause problems.
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