Right. So I'm sensing this is an issue with Windows – not Fences.
Previously with Windows 10 (/ your previous graphics card), your monitor was not disconnecting from the system when you turned it off. The monitor was off from a visual point of view, but the HDMI connection was still live saying there was a monitor there, and Windows was accepting that / recognizing the monitor as still there.
Your new Windows 11 system (/ current graphics card) is counting the monitor as no longer being connected to the system when it is powered off, which, is technically the correct behavior. It's powered off, so, you only have 2 monitors. Therefore the system adjusts to having just 2 monitors. The mouse cannot be moved on to the monitor that has been powered off etc etc.
"Desired behavior"- again it sounds like you just want those icons to be inaccessible/hidden when the monitor is turned off. I regret to inform that that's not possible. Even in absence of Fences, Windows wouldn't let you do that with icons either. If the system has been reduced to 1 or 2 monitors because some of them have been turned off and Windows is no longer recognizing them as being present, then the content that was on them needs to go somewhere.
Again your layout "with L present" will restore when L is present, so I recommend just modifying the layout for C/R-only setups such that it minimizes disruption to your workflow; stash them in a corner, etc. Alternatively, you can research ways to force Windows to continue to recognize your monitor even when it's turned off, but this isn't something I'm aware of.
Sorry we could not be of more help.