Thank you very much for the quick replies! I tried two clients, one laptop with the Intel 7265ac chipset (Just to be clear, these tests were done on 2.4ghz n). And another client with an Artheros chipset. I am not entirely sure which one exactly, I can check in a few hours once I get home. Both clients were showing the same behavior. The Intel chipset was using Windows 8.1, while the Artheros chipset was using Windows 10. As a sidenote, I will try out Ubuntu clients as well once I get home.
I initially also suspected bloated clients. But both clients showed an A bufferbloat score on an Archer C7 V2 2.4ghz wifi. This was the exact same test as the one in the OP, so with 32 upload streams. Unfortunately, I do not have the Archer anymore, so I cannot repeat the iperf tests.