I'm not sure if this is a bug or not.  but last night while playing I noticed that while the Starbase was upgrading that its health went down to 1600 and stayed there until the Starbase completed the researched upgrade. 

I also noticed one other quirky issues, it was having two Starbases in the same gravity well.  Both were upgraded two levels.  The second built Starbase was only again showing a 1600 health. but no upgrades were being done.  I kicked off a third and final level and after it completed the Starbase then reflected the correct health.

I noticed this happening a few times.  at first I though maybe this was by design and that when a starbase is upgrading it reverts to this lower health because of the upgrade.  but I then realized this has to be a bug.  Something that should be looked at.