Weird VPN issue upgrading Ubuntu 20.04 to 22.04: Firefox/Chrome +VPN combo fails to work

I ran into several weird network issue for Firefox & Chrome immediately after upgrading Ubuntu 20.04 to 22.04.
I have no clue how to troubleshoot it, or what keywords to search, so I ask for help here.

I need to use a socks5 proxy and a OpenConnect VPN (under Ubuntu 20.04).
They are proven good under Ubuntu 20.04, but become problematic under Ubuntu 22.04 + Chrome/Firefox.
The VPN connects, and curl/wget works, but Chrome/Firefox fails.

If only the VPN is used:

• Chrome fails to open most webpages under the VPN.
  Chrome opens google.com and stackoverflow.com fine, but for many other site, e.g. github.com, chrome stuck in blank loading for a while, and ends up in error 'ERR_HTTP2_SERVER_REFUSED_STREAM'.
  However, curl -vL github.com succeeds.
• Firefox cannot open any webpage under the VPN.
  The webpage stuck forever in blank loading.
  Under Ubuntu 20.04, Chrome & Firefox are fine.

If both the proxy and the VPN are used:

• Chrome & Firefox cannot open any webpage under the proxy + VPN. Both of them blank loading forever.
  Under Ubuntu 20.04, Chrome & Firefox are fine.

If only the proxy are used:

• Chrome & Firefox are working.

Package version:

network-manager-openconnect         1.2.6-4     amd64
network-manager-openconnect-gnome   1.2.6-4     amd64
libopenconnect5:amd64               8.20-1      amd64
openconnect                         8.20-1      amd64

firefox                             1:1snap1-0ubuntu2   amd64
google-chrome-stable                110.0.5481.177-1    amd64

Chrome & socks proxy does not change during Ubuntu 20.04 -> 22.04 upgrade.

Notes:

• It is almost impossible to test other VPNs, because most VPN are censored or will be immediately censored by China GFW.
• The VPN are not hosted by me, so I cannot check log on the VPN server.

Root cause is unknown, but the workaround is disabling ipv6:

sudo sysctl -w net.ipv6.conf.all.disable_ipv6=1
sudo systemctl restart systemd-networkd