Hello Jim, thanks for your suggestions, of course you were right.
On the LAN side I had a default gateway to reach some internal subnets, which tricked pfSense into thinking that LAN was actually a WAN.
I suppose that this was the reason that caused the masking of packets routed by OpenVPN and directed downstram via the default gateway.
The setting of Firewall > NAT > Outbound was and remains "Automatic outbound NAT rule generation.
(IPsec passthrough included)".
Added the proper static routes on LAN side, removed the default gateway on the LAN side, everything was back to work as expected, that is: no automatic masquerading happening for packets coming from remote OpenVPNs.
Lesson learned: the "add gateway for WAN, none for LAN" advice during setup process is there for a reason.
Thank you again
Gino