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