Did some more tests, maybe that helps a bit..
Stockcar 2018: car has road and Wet tyres. everything working as expected
Same with Eve F1: car has Bias-Ply and wet tyres: everything is working as expected
Renault Megane: car has Dry and Wet tyres. Chief pitstop wet tyres leads to dry tyres
McLaren M23: car has soft, hard an rain tyres. Soft, and hard are ok, Chief pitstop wet tyres leads to soft tyres
Reiza´s Metalmoro MR18: car has Dry and Wet tyres. everything is working as expected