The workaround would be ALt-Tab to the explorer window that opened behind your MSFS full screen window, BUT that that does not work, MSFS stays on top, which is the problem.
Workaround is to NOT be in full screen mode, so to be able to Alt-Tab, and that you do with keys Alt-ENTER. (Switching out of VR mode half of the time has horrible results).
So, before you do Space>Load, you Alt-ENTER (and/or set up your screen to be ‘Windowed’ which is hard to tell even because a maxed Windowed screen looks the same in Full screen mode, unless you use a lesser resolution, which I do on my UHD screen), and either you can Alt-Tab to file-explorer, or it pops up by itself.
I thought it game was locking up for me when I had this issue, but it’s not. It’s just opening another window to select your plan you can’t see in VR. Since I’m at my desk I just pull my headset off, select the flight plan, then click back to Flight Sim, and put my headset back on. In fact, I pretty much discovered that almost every time I thought FS had frozen, it was because something had happened so it wasn’t the active window anymore.