Disable setting DNS for a forticlient portal

We are using tunnel mode VPNs on our firewall. Now there's a subset of users that need to use another vpn client from another vendor as their main VPN while retaining our VPN client for som other uses.

I tried setting the dns-server1 / dns-server2 inside "config vpn ssl web portal" but it doesn't seem to have any effect because the VPNs still have the default dns pushed (not the ones set by me)

The desired effect would be to disable setting the DNS entirely in this case (but intermediate solutions might work)

Reading through the documentation I've found this (https://community.fortinet.com/t5/FortiGate/Technical-Tip-Configuring-DNS-servers-per-SSL-VPN-Portal/ta-p/194780) but it says that "Specifying the DNS server settings at the portal level is overriding those at the global level." and it doesn't seem to work in my case (it's because I'm using tunnel mode??)

Any ideas?