the fact that qrow calls it an "assignment" is what gets me. this is the point of time that glynda should have already arrived to vacuo with help she went looking for—by the time team rwby + jaune return, the ships from all the kingdoms are there already—so it can't be that, and if it was somehow related to this search for help... why not just say so? "tai's still looking for help" is all it needs.
but no. he has his own assignment that keeps him away from vacuo. he left patch for a mystery mission after beacon's cct tower had been broken into, and once again sometime after the breach had happened. he's got to look after "some things", an explanation why he can't go after ruby even when he wants to, why he didn't leave when yang did.
what could be more important? what requires such secrecy that it's an assignment with no further explanations?
all the maidens are accounted for with the mystery girl in the secret meeting likely being the summer maiden—and if she isn't, she's still in vacuo, not in vale or patch.
all that is left is the mystery of where the crown is, and when beacon, the place their enemy excepts to find the relic at is compromised, tai starts going on missions again; when everyone is in vacuo, tai is still in vale / patch.
he is the dragon guarding the treasure. there's no other reason to point his absence if it wasn't an important plot point.
The new rwby short reminded me of one of my favourite aspects of team rwby. Which is that it starts with Weiss and Blake being two traumatised teens with dark backstories who meet Ruby and Yang who are like wow! You guys seem to have a lot of baggage and we'd like to help with that. We're just normal sisters but we'll do our best :D
And then cut to 9 seasons later when Weiss and Blake have conquered their demons, reconnected with their families and are slowly healing meanwhile every small drip of lore we get about Ruby and Yang's family feels like this