I might be way out of line and too new to the game to provide any sort of relevant feedback....
But if you have issues with inviting friends, my friends and I noticed some similar issues with Tribes of Midgard.
Some friends couldn't join me if they were sent an individual invite, but COULD join if they went into an invite dropped into a party. Some, had the inverse issue.
Might not be relevant in this case, but may be something worth trying, since at least on PS4/PS5 there seems to be some slight difference between getting Game Invite (sent to an individual player), and 'posting' a session invite into a Party's Text Chat and having party members join that way.