I have seen this problem if a specific channel is specified instead of auto channel, using auto also unblocks max power to allow 30dBm (1000mW)
I started to use the snapshot hoping the problem might be solved but is still present. Perhaps is in the driver.
Maybe you want to try 21.02.0-rc1 Release Candidate :
https://downloads.openwrt.org/releases/21.02.0-rc1/targets/ipq40xx/generic/
Do you know why I was waiting? Because the next one I put on can benefit from the updates of the apps and re-install all the plugins. Also the proof I would like once in release is: boot to the original bank and from there try to install the upgrade. This is to see if you can do even just upgrades without losing all the various settings.And anyway, I have no problems with this snapshot.