I have, and the technical support representative at Proton confirmed it, but not without implying that it was my fault for using rclone. I asked the official recommendation for Linux users to do automated or scriptable backups onto a Proton drive and the answer was that some kind of SDK was planned for the future. Proton drive stopped working completely with rclone shortly after that, which was about two months ago.

I want to be happy with proton but their poor linux support across all their products makes it difficult.

To be honest, all consumer cloud storage providers get touchy when you access them via API.

Dropbox API refuses to sync certain 'sensitive' files like game backups (ROMs or ISOs). There is no way for Dropbox to know if you own the game and thus can own a backup, they just play file police.