Why does my trim keep snapping to maximum-up whenever I open the main menu?

Just installed Microsoft Flight Simulator (2020), and got a Thrustmaster 16000 HOTAS + rudder pedals.

Hopped into the Mount Everest discovery flight to see what calibration I needed to make to the controls.

The instant the simulation begins, the nose pitched straight up into the vertical, and stall warning started going off. After a bit of wrestling with the stick, I realized the trim was maxed out in the "up" direction. I mashed the trim down button until I was able to get the thing stabilized. Weird but... okay.

I didn't like where my landing gear toggle was bound, so I hit ESC and changed that control binding to be where I wanted it.

The instant I return to the simulation from the Main Menu, the plane pitches up exactly like before - and lo, the trim is maxed out in the "up direction" again. I wrestle it back down again, and so on.

Every. Single. Time. I press the Escape key to bring up the menu, even if I make no changes, the moment the menu closes, the trim snaps to the maximum-up position and has to be wrestled down.

I've tried searching everywhere I can, but aside from a months-old posting on the MSFS 2020 official forums where someone is apparently having the same issue, I can't find discussion of this let alone steps to fix. How do I troubleshoot this?

Running on Windows 10, via Steam, issue began with all settings at default - including the control mapping for the HOTAS.