I found my issue, although it looks like a new bug.
On linux-firmware 20201218.646f159-1 I could set the WLAN frequencies in wpa_supplicant.conf to work only in the 5GHz band as my 2.4GHz neighborhood is crowded.
$this->bbcode_second_pass_code('', 'ctrl_interface=/run/wpa_supplicant
update_config=1
country=de
network={
ssid="xxx"
psk=xxx
freq_list=5180 5200 5220 5240 5260 5280 5300 5320 5500 5520 5540 5560 5580 5600 5620 5640 5660 5680 5700
}')
This doesn't work anymore with a newer linux-firmware, I get no link. If I comment out the freq_list, I get the link but get connected in the 2.4GHz band.