This is correct, we were just discussing this earlier in these forums.
Currently, in order to gain the correct phase throughout the party, people who join must go back to Oribos (or join in Oribos) then return to the Maw.
It’s a current workaround. There is no ETA on a fix at the moment.
https://us.forums.blizzard.com/en/wow/t/why-cant-we-join-groups-in-the-maw/768295
Personally, I was saying in that thread that I really didn’t think it was much of an inconvenience at all - to the point where I was under the assumption it was just going to stay that way. But you know what they say about assuming…
Anyway, you can join a group while in Oribos, then just port to the Maw. Is it an inconvenience? Yes, but I personally wouldn’t rank it as terrible. 