WIFI works with LE9 and LE11. Do not know what causes it, but now running stable.
Don't know if it's any help, but I have been fiddling around with a Pi Zero 2 W and have had problems with both wifi and Bluetooth which are very reminiscent with those I have had on the Pi4.
I turned off the in-built Bluetooth and then WiFi worked fine. I have added a cheap BT dongle instead which doesn't seem to interfere.