Update to my last post. Today I joined that first map of mine that I talked about when there were more players already and the session started earlier (it could be 1 or 2 hours before I joined becouse it was after it got a tab in Discover). And unfortunately I did not see any Tracker UI on the HUD. Then I joined the same session also with my second account from different computer and it was the same (no Tracker UI on the HUD). So at this moment I have no map where the Tracker UI would work correctly for Joined In Progress players. It is either visible only for some Joined In Progress players, or for no one at all.
One additional info: about an hour after I joined that session with my 2 accounts and I didn’t see any Tracker UI on the HUD (which I described above) my friend joined the same session from different location and different computer and the Tracker UI on the HUD was visible for that additional account. Of course there should be some specific reason for it, but it just looks random so far.
And to clarify - I am aware of long-term issues with rounds, so all of my islands don’t use any kind of round system at all.