Mvebu broken Ethernet Flow-Control?

I'm on master version OpenWrt SNAPSHOT r13919 with kernel 5.4.52 and i see a weird bug. If i keep the default ethernet Flow-Control RX/TX enabled my upload speeds drop from my max 40 Mbit/s to 10 Mbit/s. downloads are ok.

If i just disable Flow control in the Win10 Ethernet Adapter uploads go back to normal, if i enable Flow-Control they are back to 10 Mbit/s. This is independent of the machine, so a other PC behind a unmanaged switch with enabled Flow-Control also drops back to 10 Mbit/s. If i directly connect the system and also disable Flow-Control uploads are back to 40 Mbit/s again?

Any idea whats going on and is there a way to disable Flow-Control for all LAN ports from the openwrt side, since i can't disable it for the unmanaged switch?