Hi,
Got this very same issue. Moved a from working with v6 (ovpn) config from 2.2 (yeah, old !)
to 2.4.2, and reconfigured openvpn.
Before with the same settings in 2.2 I got everything (including openvpn v6) working now,
I got in the (same as you) situation where I see packets over v6 coming to the openvpn link,
but no reply from the (outside) net, while I set rules on the ovpn interface to allow both v4 and v6.
I have the tunnel interface net defined as a /64 from my providers /58.
V6 routing on non-openvpn interfaces works great !
Do I need a static route to the ovpn interface maybe ?! (not needed before)
It might be due to the fact that the prefixes in the /58 that I use in the client subnet have not
explicitly been requested by dhcpv6 or so ? where before this just worked..
(note, I only changed the version of pfsense, nothing else)
Related question, how do I tell the dhcpv6 client to request that specific prefix as well as the others
that are distributed through the wired interface (ipv6-follow)
Rudi