As a reference to the issues in this thread and a workaround from this post I've tried the following.
I've just returned to the latest (at the moment) Ath11k firmware WLAN.HK.2.9.0.1-01862.
Rebooted the router and tested once again without multi to unicast option. Confirmed the loss of both Ping and Iperf3 traffic.
I've turned on multi to unicast option only for the 5G radio.
In wireless config it is option multicast_to_unicast_all '1'
I've tested once again with multi to unicast option turned on.
There is no more loss of both Ping and Iperf3 traffic if a client disconnects from the 5G WLAN.
There is a long discussion commenting this issue.
So I suggested in my earlier post these issues were connected. That is now confirmed, I think.
@egc, @asvio, @Catfriend1 @Pow maybe you can try to use the workaround enabling multi to unicast option in wireless settings.
Maybe developers can find the real cause and resolve this.
@robimarko, @Ansuel, @nbd, @kirdes, @hnyman, @quarky