Hi Blobbie01,
Finally, I found that mwan3 impacts wg client & ipv6.
More effort is needed to construct a suitable rule for this situation. To temporarily make it work, use changed wgclient-route-update.sh and disable mwan3, your previous script should not be run.
/etc/init.d/mwan3 disable
uci set mwan3.globals.enabled='0'
uci commit mwan3
reboot