VTOL plane fly away in multicopter position mode

This week, we have been testing a VTOL aircraft in Multicopter Mode.
We set-up a small mission to test some parameters and perform a rejected transition just to make sure that everything works well before going for the full transition.
On Wednesday we performed several flights without any major issue, just some issues where the plane didn’t achieve the correct heading and aborted the transition.
However today we were doing some tests and the plane started to behave weird and seemed a little bit unstable in Multicopter Mode. The worst happened during the last test when the drone started to fly away and even when we switched to position mode it wasn’t responding to our Roll and Pitch corrections. We managed to land the plane safely but we don’t have a clue why this happened.

I attach 2 logs, the first one was one of the logs obtained last Wednesday when there was no major issue:

The second one is from today when we have the fly-away issue:

Thank you in advance for any ideas.

From the log, the velocity tracking diverges. But the throttle output is almost always in a saturated state. Is the weight of the drone increased compared to the normal flight? From my point of view, I think you need a stronger power system

No, it’s exactly the same vehicle configuration used during the flight of Tuesday (When it flew well).
If anything, the weather was slightly more windy.

I agree that the plane/drone is a little bit heavy but this is the first time we observe this behavior.