Cabin pressurization with high landing elevation

Hello,

I set a landing altitude of 6900ft. I was held low during departure and the cabin climbed at about 500fpm to 6900ft. This is exactly right as described in the FCOM.

However, at about 20.000ft during climb, the cabin rapidly depressurized:

This is very unexpected, as according to the FCOM, it should climb to the maximum of the theoretical cabin altitude and the landing elevation and not descend again during climb. It appears that it tried to follow a delta P of 8.1 during climb?

Also, how can it depressurize with all outflow valves completely closed? In that case, I would have expected the cabin altitude to climb.

Descend is also unexpected. Again, it should descend to the higher of the theoretical cabin altitude and the landing elevation. Instead it descended way below the landing elevation at a rate of up to -1500fpm(!):

At some point it climbed again to the selected 6900ft. It does not look like the cabin pressurization controller is implemented correctly.