I've noticed using Excel (single window with multiple taskbar icons) the rightmost tab of that program will shift to the other taskbar (even though the window is on the current window).

This is with 'Taskbar Mode' - 'All taskbars show relevent windows'.

The taskbar icon can move even if the window is stationary, it will move when the particular icon is focused but will move to the next monitor when not in use.

If this is possible to track with the debug or something let me know and i can try capturing some events.

Sorry for the delay, i wasn't back on this computer until today. Still occurred, see attached for log.

From recent restart it still occurs. Requires a windowed window (not fullscreen).

I can describe it further if necessary but hopefully the log will outline the changes better.

Correct, all on 1 monitor but whenever not in focus one of the taskbar icons moves to another screens taskbar. Where i'm usually using it i have a 3 monitor setup - Laptop screen & 2 external monitors. Typically this issue moves the non-focused excel icon to the next monitor (so if started on 1st external monitor, goes to next - if started on main laptop it goes to 1st external).

All this moving occurs only when the actual program the taskbar icon represents is not in focus, if it's in focus it will return to the correct window.
This happens when the it is restored or minimized (not maximized).

Weird. Still happens, since i'm more of a keyboard user than the mouse it isn't a showstopper - but still weird.

It requires 2 as far as I've noticed. Often day to day if i'm using more than 2 in a single window it's on my rightmost monitor and won't move (even with this issue), as there are no further monitors.
This bug is within 1 excel window (multiple files inside one window), while i use multiple instances sometimes, this issue is for a single instance.

I've found a little easier reproduction on my system you can try. Open 2 excel files on a center or left monitor (center in software configuration too) and align right the excel window. One of my taskbar buttons moves to the next screen automatically, whichever the one that isn't in focus.

I hope this helps you determine where the issue is. Please let me know if you need further information, logs, or other answers.

Glad to see you find replicate the bug. I was starting to think it was just me.

Thanks for keeping me in the loop, i look forward to seeing the fix.