USB tether (`urndis(4)`) not working

Hey everyone,

I decided after many year to try again to slap OpenBSD on my old Acer Aspire one. The Wifi adapter is not supported, but I was thinking about using my phone to get network connectivity out of it. Unfortunately I'm hitting a strange behavior: ifconfig urndis0 autoconf works as expected, and I get an IP over DHCP. Ping and DNS resolution work as expected, but when I try any TCP connection, the connectivity simply stops working, and I cannot even ping the gateway anymore. dmesg doesn't say anything about it.

Has anyone encountered this already ?