Party member HP displaying as ??? when standing right next to them

[Umo]

We found the only solution was to disband the party and re-invite, which solved the problem, although it created complications mid-guildleve.

I've experienced this issue long ago before the first patches. We found the solution was to go away from the party untill you see ??? on all party members, then come back. This may fix your problem while waiting for a real solution from the dev team. Hope this helps^^