It basically seems to work. I received a test APK (from Krish Jha - Niotron team) that copies a text file from the assets to /Download or /Documents (of course without WRITE permission, which is no longer available on Android 11).
/Downloadand/Documents
are the ONLY two folders where this works. (even /Downloads does NOT work!)
Tested on a Pixel 2XL (Android 11).
I don't think this has anything to do with SAF.