Arduplane 4 Massive Altitude Drift when stationary

It’s my first post here so Hello Everyone.

I’m trying to find a resolution to the misterious issue I came accross related to the altitude reading of my Ardupilot, and before I blame it on the board I wan’t to ensure I didn’t miss anything obvious.
The board I’m using is the Mateksys F765 Wing, brand new, never in use, running arduplane 4.0.8, downloaded from mateksys.

When connected, upon the startup calibrations and init of the sensors, the altitude reading starts sinking into the ground by enormous numbers. In arround 20 minutes it can get down to -300m, it doesn’t seem it ever stabilises, but eventually slows down to a meter or so a minute.
I noticed than when powered via USB only, the descent speed is slower, but still present. I thought this might be temeperature related, looked for some baro temperature compensation, but concluded that this wouldn’t account for such magnitude of error.
I run the bare board, no auxilaries connected.
Do any of you have an idea of what might be going on? Any tips on what to look for?

It turned to be the board in the end. Noticed that the baro raw readings are completeley not where they should be, in the region of 500 mbars as opposed to arround 1000. Blowing over the baro wasn’t changing a thing. With not much to loose, as the board even after soldering the connector pins is not returnable - decided to give it a go and replace the BMP280 with one I had from another fried controller. Bit of hot air and voila - all working.

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Good fix. Lucky you had a spare. I rendered a F405-CTR board useless (for ArduPilot) this fall when I though I’d be a smart guy and put conformal coating on it so I could fly in the snow. Turns out I covered the barometer and ended up with the same problem you described. No good deed goes unpunished.

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