Buster comes with two iptables flavours
update-alternatives --list iptables
/usr/sbin/iptables-legacy
/usr/sbin/iptables-nft
By default it uses iptables-nft. When changes to iptables-legacy
sudo update-alternatives --set iptables /usr/sbin/iptables-legacy
it works with UFW without problems after reboot.
https://raspberrypi.stackexchange.com/questions/100598/ufw-and-iptables-on-buster