AFAIK you can't disable pmf with wpa3, its obligate. Only with wpa2 you can set it on or off. So if you set pmf off, you will connect with wpa2 only. Latest driver for rtl8812au should support wpa3 sae, but seems not wo work (SAE WPA3 authentication does not work · Issue #761 · aircrack-ng/rtl8812au · GitHub).
Btw, my rtl8812-based stick now works, connects with wpa2, don't know if router uses pmf or not as it does automatically.
On my FritzBox 7530 PMF setting on/off is only available in wpa2-only mode. With wpa2+wpa3 mixed mode its enabled as default and you can't disable it (activated but greyed out). So maybe router disables pmf automatically if connected device doesn't support it, but thats nothing i could set.
Next info: Tested again and found out that connection problems here seems to be router-specific. Changed settings on my fritz-box 7530 from wpa2+3 to wpa2 and back, suddenly all works again. So device is ok, driver is ok and libreelelec works. Connection works with wpa2, router setting is mixed mode. Thats why i normally hate wifi, too much problems with some devices and hard to figure out what's the problem 
nihilista Chewitt's build connects to an AP with PMF as optional/enabled for you? You were talking about WPA3 which is separate from PMF, isn't it? It actually doesn't work with PMF as optional and WPA2 aswell for me.
With onboard wifi connecting never was a problem. But with my usb wifi stick still no connection (Input/output error). But under windows i'm not able to connect, too, so maybe bad device or some router issue. I have another stick, but unfortunately it needs rtl88x2bu driver, so i couldnt test it on libreelec. I hope that driver will made his way in libreelec soon
Tested it, onboard wifi connects fine as before, usb wifi not. But i wasn't able to connect usb wifi under windows, so maybe the stick is dead or problem is between stick and box. Will have to test with another stick, but the only i have is rtl88x2bu which isn't supported for now.
I did a little research and found possible cause: libreelec uses connman for connections, but older builds of that didn't support wpa3/sae or mixed mode. I think onboard wifi didn't support wpa3, so it automatically uses wpa2 and that works. Seems connman/connmanctl could manage wpa3/mixed now (found that [RFC connman v1 1/1] gsupplicant: Add support for WPA3-Personal - iwd - Ml01.01.Org) maybe that could do the trick. Btw, don't know why onboard wifi connects and other not, as it didn't support WPA3/sae too afaik. Maybe its worth integrate newest connman....
I never had problems with connecting RPI4 onboard wifi to mixed mode router (fritz box), even with LE10 it connects fine with wpa2.
But my realtek wifi stick (rtl8812) fails to connect, i think that starts somewhere between LE9 and 10, after that i switched to xbian. I found usb wifi was bit more stable than onboard.
Back to LE10 my hope was that its fixed, but no. Btw, maybe its a driver or router issue....
