Wifi phone crashes/drops off the network with 21.02.0-rc1

I have an old SIP wifi phone (Siemens SL75WLAN) that doesn't work well with openwrt 21.02.0-rc1. The phone connects successfully to the network and seems to work well when idling or handling little wifi traffic (such as SIP registrations). But, if I make/receive calls or if I access the phone's web interface the phone seems to drop off the network (the phone no longer responds to ping).

I don't see any messages in the router's logs (kernel and system log). The phone doesn't show any indication of error.

I've seen this problem with two different routers (Netgear R6220 (mt76) and D-Link DIR-825B1 (ath9k)).

I realize that this may not be an openwrt bug.. The phone is old. It's a 802.11g device which only handles WPA1 with TKIP, but it works well as a SIP phone so if I could work around the problem that would be great!

Does anyone have any suggestions on configuration changes I could try to work around this? Are there any specific wifi settings that have changed default values between 19.07 (which works well) and 21.02? And if you think I should file this as an openwrt bug, let me know and I'll do that.

(I think "Allow legacy 802.11b rates" has changed default value in 21.02, but I've had that disabled with 19.07 also.)

My wifi configuration for the phone network is below and I have not installed any additional openwrt packages after upgrading to 21.02.0-rc1.

config wifi-device 'radio0'
        option type 'mac80211'
        option hwmode '11g'
        option path '1e140000.pcie/pci0000:00/0000:00:01.0/0000:02:00.0'
        option country 'SE'
        option cell_density '0'
        option htmode 'HT20'
        option channel '13'

config wifi-iface 'wifinet2'
        option device 'radio0'
        option mode 'ap'
        option ssid 'telephone'
        option encryption 'psk+tkip'
        option key 'password'
        option network 'lan'
        option wpa_group_rekey '0'