So, this is my problem: after some inactivity period (when everything worked just fine), I needed my pixhawk for a new task. So I've updated QGC to 3.3.0, connected pixhawk, get to airframe selection and seen rover options there.
Decided to update px4 firmware first. Updated, get to airframes tab and boom, no rover there. I thought it may be deactivated for new FW, downloaded source. Checked airframes list and rover (axias and traxxas) is still there. Tried to reflash few times, but with no luck. Reinstalled QGC, no luck. Found this issue - https://github.com/mavlink/qgroundcontrol/issues/5792 - that with the new FW boards with 2MB flash will not work correctly with px4fmu-v2 FW. Btw, my pixhawk detects as px4fmu-v2 ver 4 board id 9 flash 2mb. So I took old px4fmu-v3 fw, flashed it with QGC and yey! rover appeared. Reflashed it back again wit newest FW ( I thought it will select one based on previous version, but pixhawk still detects as before, and qgc downloads px4fmu-v2 FW). But, despite of "wrong" FW, romer airframe is still there after few reflashes and disconnects.
So, the question - how to make my 2MB pixhawk to appear as px4fmu-v3, not px4fmu-v2? Bootloader reflash?
Now, the rover is gone again.... And flashing v3 FW stuck at a near end...