In my current game of .81 I have unlocked the medical center improvement. I noticed that I can now build a medical center and a colonial clinic on non capital core worlds.
The medical center is a straight upgrade to the clinic however due to the fact that in GC4 you can only upgrade districts and not colony improvements this is probably not a bug but by design.
While I like that the new approach results in less "building upgrades" having 2 separate pop growth buildings seems clunky.