Ok, I'll ask and share what I learn. Thanks!That question is better suited for your vpn provider since they know the technical details of their vpn hosting setup.
You got correct response because loopback addresses are not routed.This is what I got as an answer:
"Your Debian system port on your local network won't be able to be seen by other people using the same VPN unless you provide them with your port number."
I decided to change my rule to allow SSH from 10.20.30.0/24.