Here's what I posted in a Beta thread..
This is when you mouse over a ship in the fleet view port in the lower left.
It usually shows:
Class
Hit Points
Moves
Sensor Range
Logistics
At a random point it starts only showing:
Class
Sensor Range
Logistics
Completely missing the 2 most important pieces of information.
The only way to see those pieces of info is to take ships out of the fleet, view their HP / Moves, then put them back in - tedious.
Alternatively I can quit the game completely and reload it.
Now I have found the cause - It's simply zooming in / out with the mouse wheel!
3 times I just loaded my game, zoomed in and out a few times, testing the mouse over every few secs and every single time the UI failed after 2-6 zooms!
One fail was after only 1 click of the mouse wheel zooming in (at quite a distance out - it doesn't seem related to going to detail view mode).
Both HP and Moves vanished from mousing over a fleet.
I hope there can be a fix now because it's REALLY annoying constantly reloading the game to deal with it.
Edit: Ok forget that, there are multiple causes.
I just had 1 fight after loading my game and it did it then too 
I literally can't play more than a few mins now without this happening - I'm gonna have to give up until this is fixed..
That info is absolutely essential when in a massive multi-fleet war.