Just for good measure and for testing purposes:

AFTER taking notice of "does this happen on taskbar on both screens or just the one", I also recommend turning all 3 of these settings off and trying it out further:

And turning this one on. (Which, I recommend turning this one on in any event!)

Thanks for your help diagnosing.

Sigh alright. So there's the bad news.

There is something on your system that is messing with the Work Area on the system. Those numbers should not match.

https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-systemparametersinfoa#:~:text=primary%20display%20monitor.-,The%20work%20area%20is%20the%20portion%20of%20the%20screen%20not%20obscured%20by%20the%20system%20taskbar%20or%20by%20application%20desktop%20toolbars.,-The%20pvParam%20parameter

https://learn.microsoft.com/en-us/windows/win32/api/winuser/ns-winuser-monitorinfo

What that test did was query the system's monitor sizes, and then it used SPI_GETWORKAREA / GetMonitorInfo to get the "rcWorkArea" for the monitor in addition to the "rcBounds". Windows maintains (and one can set) the work area -- which per the docs is the area in which content should confine itself -- i.e. the area "minus the taskbar". Back in Windows 98 through Windows XP, you could also have additional bars on the top/left/right/bottom sides of the screen, too. We even had an old program ObjectBar that did some stuff like this In Windows 10/11, this is predominantly fixed. Apps can modify this, but none do because frankly none should, and the https://learn.microsoft.com/en-us/windows/win32/shell/application-desktop-toolbars framework is something that has been obsolete for a long time.

Something on this system is causing the work area to not be set up correct. And unfortunately, I just don't know what that is :/ But I encourage checking out other apps that are installed and using that test app until they're different. Sometimes monitor apps like DisplayFusion can do stuff like this.

Looking forward to learning what you find but unfortunately, this one is out of our hands for now.

Going to the Microsoft driver made no difference unfortunately. I had one monitor on the Samsung driver and the other on the Microsoft driver. The funny thing is that all I did what swap out the monitors. It has to have something to do with that.

An interesting approach would be to have an option in Fences to tell it that there is a task bar or no task bar. Perhaps even a way to move a fence to the taskbar and tell fences that this is where to initiate the rollup. It is very elegant and automatic now, but perhaps a manual setting could help when it is not detecting properly.

W00t! Yes those are exactly the numbers I'd expect. (But yes the solution does not make any sense. But hey sure.) Glad it has gotten fixed!