I can add that in my 2x2 monitor arrangement, the panel is reserving space but on the wrong monitor. If the panel is on a monitor on the bottom row, it reserves space on the monitor directly above it instead of the monitor that it's on. Seems to be a variation of @fbernier comment.
On my variation, the panel is at the top of the bottom monitor, and panel is reserving space at the top of the monitor above it. If I put the panel on the top monitor, the behavior is what is expected.
However, if I put the panel at the bottom of any monitor, it does not reserve space anywhere.
I can add that in my 2x2 monitor arrangement, the panel is reserving space but on the wrong monitor. If the panel is on a monitor on the bottom row, it reserves space on the monitor directly above it instead of the monitor that it's on. Seems to be a variation of @fbernier comment.
On my variation, the panel is at the top of the bottom monitor, and panel is reserving space at the top of the monitor above it. If I put the panel on the top monitor, the behavior is what is expected.
However, if I put the panel at the bottom of any monitor, it does not reserve space anywhere.