I find this an interesting (point/observation) considering I doubt very many of us forum members truly are Wireless Network Engineers.
Perhaps it does happen on occasion... That an advanced feature isn't really doing anything (or what it's supposed too).
And although certain users enabled a feature with good intent, everything works for them as expected... UNTIL a more recent firmware upgrade fixes the feature.
Myself, I am NOT a network engineer... but I have a gut feeling...
-This may have happened to some extent with the Enable 160 MHz Channel bandwidth feature?
And perhaps... It's currently happening with WMM & WMM APSD (PowerSaving)?
But I'm just theorizing.
I'm a retired network manager so I know a bit. I'm not running the beta yet I have the following firmware:
Router: RT-AX-86U firmware 386.7_2
Node: RT-AX-86U firmware 3.0.0.4.388_21709 (Native code)
Node: ZenWiFi XD6 firmware 3.0.0.4.388_21380 (Native code)
I use 80-Mhz wide channels due to airport proximity in the US.
My WMM settings:
Enable WMM - Enable
Enable WMM No-Ack - Disable
Enable WMM APSD - Enable
I believe the above is the default.
I disable Airtime Fairness, it can lead to strange issues of the type described. I'm also suspicious of your 160 wide channels.
I have 802.11ax/ac Beamforming enabled and Universal Beamforming disabled.
I have no reported drops and no slowdowns. My nodes have the newer code, so I think you and the others should be looking at settings. Something may have change between versions that's affecting your setup.
If you are near an airport or see or hear aircraft regularly you should disable DFS and then there will be no 160 wide or at least there should not be.
As a general rule for configuring, never change a default unless you have a good reason to do so and this means understanding what the setting dose and the implications of the change.
Good luck!