WAX620 5Ghz stutters & ping loss on 5Ghz client disconnect with multiple SSIDs and VLANs

Running OpenWrt SNAPSHOT r26467-50e26e6fd7 on two WAX620 APs and both are doing the same behaviour. I was also observing this on previous versions, inc. stable, so went to the this snapshot.
Have 3 vlans configured. One on 2.4Ghz SSID and two on 5Ghz SSIDs.

A client connects to each of the 5Ghz SSIDs and runs a ping to the SSID gateway for that VLAN and all is well. But when a client disconnects (turn the Wi-Fi off) from the 5Ghz SSID the traffic stutters, a ping is lost (or the reply time is massive >500ms or even higher) and then carries on.

If I repeat the test on the 2.4Ghz, it's fine and works perfectly. If I only have 1 SSID in on the 5Ghz it's fine with clients disconnecting, no packet loss or stuttering.

If I change the two 5Ghz SSIDs to be on the same VLAN, I get the same stuttering.

If I ping the device from a wired connection whilst doing the above tests it's absolutely fine. It's only losing packets and stuttering from the 5Ghz wi-fi

I can't see anyone else having this issue, and wonder if it's a driver problem? And if there is anything that can be done?

For now I've left only 1 SSID on the 5Ghz and 2 on the 2.4Ghz to see how things go.

I'm wondering if this is related link will try the fix tomorrow

Edit: This made no difference - reading Abnormally high latency of wifi with VLANs and Ath11k possible firmware bug - traffic interruptions when a client disconnects from WLAN - #38 by asvio seems this is an issue that no one has really gotten to the bottom of. For now the workaround is to only have one SSID on the 5Ghz adapter

Please connect to your OpenWrt device using ssh and copy the output of the following commands and post it here using the "Preformatted text </> " button:
grafik
Remember to redact passwords, MAC addresses and any public IP addresses you may have:

ubus call system board
cat /etc/config/network
cat /etc/config/wireless
cat /etc/config/dhcp
cat /etc/config/firewall