Well you gave me and idea, I did have these 2 rules, both have been in place a while:
deny 0 10.0.1.0/22 1-65535 (Preventing any device from taking ALL external ports, including overriding other port forwards)
deny 3074 10.0.1.0/22 3074 (there is a reason and xbox never had a problem before)
I deleted both, of course UPNP restarted (which I had done before a few times manually).
Then on the Windows 10 system restarted iphlpsvc (IP Helper) which is responsible for teredo and UPNP for it, and it registered its port fine, re added the rules restarted the service again, and again it registered it fine.
Tested it a few times with and without the rules, seems to work either way now.
No idea why that worked but it did. Thanks for the idea.
Ill keep and eye and report if it stops working again.