ESYNC/FSYNC causes Uplay to fail to upgrade

Your system information

• Steam client version (build number or date): Feb 1 2022
• Distribution (e.g. Ubuntu): Manjaro
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

Tried two games (Assassin's Creed Origins and Odyssey) In Proton-Experimental
Both needed a Uplay upgrade.
Disabling ESYNC and FSYNC allowed the upgrade to succeed immediately after trying several other things.

Uplay launches fine with them enabled unless an update is needed.

Others have reported it can update with them enabled, but it may take several tried, seems to be luck/random.

Steps for reproducing this issue:

Download a fresh Uplay game/clear compat data.
Try to launch the game and Uplay will attempt to update.
It may work, it may not.
If it doesn't it is usually consistent and disabling esync/fsync will make it work right away (I did not try it individually)

Start by reproducing the Uplay upgrade with a fresh game or cleared compat data in Proton-Experimental on Manjaro, comparing ESYNC and FSYNC enabled with them disabled. The issue is resolved when Uplay upgrades reliably without requiring those options to be disabled; no source files or tests are identified in the report.