Why does Tixati v3.29 randomly say "DHT: port unconfirmed" on the bottom status line and other than exiting/restarting Tixati how do I fix this problem?

Tixati hasn't RECEIVED any incoming connections on this port yet. Peers cannot connect to you, only you can connect to peers who have avoided this issue. Your connectivity is impaired.

This is either due to
1. firewall/AV blocking
2. port-forwarding not working
3. ISP doesn't give you an exclusive IPv4 (aka white IP) neither IPv6 to use
4. you are connected to a VPN (see 1 & 2)
5. You use a proxy for Tixati (you shouldn't)

I use Tixati v3.29 and NordVPN on three computers and ONLY have that problem on one of them.

For NordVPN (argh) refer to points 1-3.
I haven't tried to understand the logic of usable ports when behind a VPN. All I can say, without a firewall interfering the rate of success is spurious at best. It is one of fhe reasons you would want configurable and working port forwarding.