I also have issues with iPhone 13 and Redmi AX6S (similar to the devices in this thread - thanks @jkdf2 for directing me here).
I’ve found that AX performs worse than AC specially with iPhone but with other devices as well (see here). For now I disabled AX (using only AC) and everything works great.
I understand that AX in OpenWrt is still under development and not mature yet (even if it’s supported in the 22.03 stable channel).
I’ve noticed that the master snapshot releases had the mt76 drivers updated. I may try it when I have some free time to check if AX performance has improved.