Now I dried it for 19 hours in my Sunlu S2 @65°C. I weighed the spool before and after drying. Before drying it had 1191,6g. After drying: 1191,6g. So no measurable loss here. And the next test-print did look the same.
Do you still think it is wet filament? What else can cause this?
I found something similar here and here a more extreme example. Unfortuntately, without any real solution mentioned.