Well, the problem is in the title. Tixati fails to use port forwarding in VPN and tries to forward on the router.
I am using Hide.me VPN, which has dynamic port forwarding. But after much frustration and many experiments later, it became clear that Tixati can't open a port on my VPN's interface where qBittorrent does it effortlessly. I am a long-time user of Tixati, and it is very problematic for me to switch to qBittorrent as I have more than 140 torrents active.

I have, however, found a workaround. And that is to use a portmapper software to manually open a port for Tixati and then use Tixati. This is a very ugly situation, and I hope dear dev will look into this.

Along the way, I found out that this is a very long-running problem, and it also has a side effect. If the port is not manually opened this way, then the DHT also fails to work after a minute or so.

So please look into this and fix what is wrong. As qBittorrent is open-source, a comparison should make the solution easy.

Thank you for your time.

There is very little technical information.

Service website says that offer UPnP for paying clients. Is that what you use?

How do you check that the port is forwarded, and incoming connections are present?

Do you bind to the specific VPN network interface in Tixati settings? Or do you rely on some kind of magic performed by VPN client application to send all the traffic trough VPN?

What does Tixati log tells you about UPnP forwarding attempts? There is also an extensive UPnP log in the same Help — Diagnostics submenu, you can save it, and send to the author (if there is nothing too private you see in it).

xldevelopment.net/upnpwiz.php
This is the app that I am using right now. The procedure is that I turn the VPN on first. Then use the app and open the port for both TCP and UDP using the plus sign, aka the add port forward button. In my case, my VPN lets me use dynamic ports, so I open the port that I set in Tixati. Now open Tixati and from Connection select VPN interface. Close the settings, and you are done.
Now the external peers can see my client, and I can see my open port from canyouseeme.org. But without doing this, the port is not open from the outside, and no peer can see my client as if I have a strict NAT and closed port.
I don't know what part of the log is needed or what detail level needs to be set to see the issue. So I didn't upload any. If anyone has an idea, then let me know, and I will upload the logs.

Well, but do you have UPnP port forwarding turned on in Tixati at all?

It doesn't work either way. And when using a VPN, you are not sending a request to a router, so all VPN providers ask you to turn off "UPnP forwarding on the router" option.

In the last month, I have tried all possible combinations, and only the Above system worked for me. This workaround was found in this very forum, and that post is more than 2 years old.