After the comment from tatesn I tried out the following:
I have 3 PCs (1,2,3) and 3 microsoft accounts (A,B,C) and an XBOX X
if PC 1 with account A starts a game the other two PCs cannot see it (PC2-B and PC3-C)
if PC 3 with account C starts a game the other two PCs cannot see it (PC1-A, PC2-B)
if PC 2 with account B starts a game the other two PCs can see it
if PC 1 changes the account to B and starts the game the other two PCs can see it
if PC 1 changes the account back to A then its broken again
if account A starts a game on XBox X account C cannot see it
if account B starts a game on XBox X account C can see it
The difference (what I know of) between accounts A and C vs B is that A and C was always an adult account, while B used to be a child account then I got upset with MS restrictions and changed it to adult (but it is still somehow a mixed thing in MS) it seems I remembered wrong, all three was an adult account. So it seems to me it is really depends on the MS account starting the game.
I have two win10 machines, both are running 1.19.62 build 13844480 branch r/19_u6, one starts the server the port locally is not opened and there is an ICMP message destination not reachable port not reachable. If the other starts a game, then all works fine. The firewall is off on the problematic machine.
I can confirm, it started to work without any change I know of.