I reported this to Realtek and they pinpointed it to a bug in wpasupplicant.
I tested the patch and it works fine for 8812au, 88x2bu and 8821cu. I then reported it to launchpad in case it makes it for Ubuntu 20.04: https://bugs.launchpad.net/ubuntu/+source/wpasupplicant/+bug/1867908If others here test that oneliner-patch and find that it solves this issue, maybe we can just report it upstream and to wpasupplicant in debian, not in network-manager.
