Cannot use a mounted SSHFS drive as a Steam games library: "Failed to add new Steam library folder"

My current setup:

• I have a Raspberry Pi 4 running as a home server, with a 2 TB external hard drive mounted as permanent storage.
• Then I have a Windows 10 computer with Steam installed, my plan being to place rarely-used games on the server storage to save space on my SSD.
• I created a folder on the external hard drive named "backup", and set it to be chown'd by the admin account, with full read-write permissions (chmod 777).

I mounted the device using the following command and assigning it to drive Z:

\\sshfs.r\admin@192.168.0.100\mnt\7ad490a4-ab4d-45fd-82ea-10240409b7e6\backup

However, whenever I attempt to make Steam use the new drive as a games library, the process fails with an error message: "Failed to add new Steam library folder". I checked whether the drive was in read-only mode, it doesn't seem to be the case as I can create and delete files freely. Is this a problem with Steam itself, with the SSHFS implementation, or both? And how can I get a debug log from either of them so I can check what's happening behind the scenes?

The issue names no repository files, tests, or entry points. Start by reproducing the SSHFS mount with the provided command and then attempting to add drive Z: as a Steam library; done means determining whether the failure is in Steam or SSHFS and capturing useful debug logs.