I believe I found your issue, as I encountered the same
Into dmesg, I have the following message that appeared after few hours:
[11310.260222] raspberrypi-firmware soc:firmware: mbox_send_message returned -62
[11310.267387] raspberrypi-clk firmware-clocks: Failed to change pllb frequency: -62
I didn't encountered the issue on the rev 1.4 board. I'm now using 5.4.96 kernel on the rev 1.1 board.
I cannot access the cpu frequency anymore, as the command
cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq
Is hanging (and cannot be killed). At Full duplex, I cannot receive more than 600 Mbps while sending 400 Mbps (I was able to do much much more yesterday) : I believe the CPU frequency is stuck because of that.
However with packet steering enabled, I'm still able to reach ~1000 Mbps at 1 single way at a time.
Its clearly another issue (at this level this is probably related to hardware/kernel/firmware), so may be creating a new thread will be more convenient.
EDIT : this thread seems to be talking about the issue