I'm afraid I must report that the new update (11.0.6, Beta 6) didn't change anything with the wallpaper issue, at least on my PC. Changing the wallpaper still does nothing, the prior wallpaper selection stays on my screens. This is the case whether that is done by bringing up the DF menu in the system tray and selecting a pre-existing wallpaper profile from the "Desktop Wallpaper Settings" menu, OR bringing up the full Desktop Wallpaper window and creating a brand-new wallpaper profile with different pictures and then hitting "Apply" and "Okay".
As I mentioned in the OP, sometimes the wallpaper would
eventually change suddenly, an hour or even multiple hours later; for today on Beta 6, I don't know if that's the case or not just yet, as it's been about 1 hour and no change yet.
One other bit of info: I updated to Beta 6 then immediately tried to change my wallpaper. When nothing happened, I restarted my PC. Upon restart, the wallpaper change I had made DID take effect. So a restart does (perhaps unsurprisingly) "force" the wallpaper changes to take effect, despite whatever problem/glitch is going on, but
until I restart, no changes will take effect (barring the random "sometimes the changes will suddenly happen hours later" phenomenon). Note also that Display Fusion "thinks" the changes have been applied - if I open the menu and go to "wallpaper", the new wallpaper I selected shows up in the window as if it HAD been properly applied, even though the
previous wallpaper is still present on my monitors.
Let me know if you need any further info or logs from within DF, etc.