Max-Zorin
I'm new to GrapheneOS. Have been taking it for a spin on a pixel8a for a little while now.
Both Syncthing and Joplin are on my top list of apps.
I've stumbled upon this problem as well. So far, Syncthing has been working the way i'm used to.
Problems started to show up when i tried syncing a Joplin folder, holding hundreds of small .md files. Syncing took hours...
After a little research i think i found the cause. Unfortunaly, no solution yet.
The real issue here is about changes to the Android filesystem (Android filesystem abstraction layer), as you can read here.
This applies to Android 11 and newer.