I've had this issue also. Additionally, most of the time my dinos don't show up in the tribe list, or party list. There may be a lot, few, or none, but i've never had all my dinos listed.

To expand on what Havoc said above, it seems sometimes dinos just drop from the party OSD. Initially, I thought it was a range issue, but I can't be certain of it.

Same here, I lose the list for the party and tribe dinos, all but the one I am riding, after about 30 seconds or so. I have to log out and back in to reset the list. However, even though I can't see them the party commands stil work for the dinos in the party.

Also lose the ability to see their health bar.

I'm working on this issue at the moment. Is a complicated problem to solve. Basically Ark puts dinos that it considers to be out of range or not relevant to any players into stasis which causes them to be invalid on the client side.

Thank you Milow, you are the best.

Milow. The author of Homing Pigeon had a similar issue which caused flyers out of render boundaries (ie. in stasis) to stop flying to his Homing Pigeon Flag. He fixed it. You may want to try and contact him.