I've been speculating for a while, that if and when they release an updated mini, that's also going to be when we see another Apple-branded display, which we know they're working on alongside the new new Mac Pro ... because they said so.
But, considering there aren't the same kind of time constraints in releasing a decent display (whatever updated iMac display minus the computer, basically — I know it's a bit more than that, but not like a new Mac Pro), it could be ready way in advance of a Pro.
And — and this is key in my mind — I cannot imagine them releasing a 2018 mini after nearly four years and no display to sell alongside it. They're not going to show it off with the LG UltraFine, are they? This presents a pretty lousy situation for Marketing, not to mention the upselling potential on the trillions (if the level of whining in forums is to be taken as an indicator) of new mini purchases coming when they release one.
Keep in mind they still have the discontinued Thunderbolt Display shown here — kinda weird:
https://www.apple.com/mac-mini/