I started a new game under 1.71.495432 and immediately ran into an issue. I did not like where the Capital City was located so I deleted it with the intention of putting it in a different location, but things did not go as I expected.

The first screen snapshot shows what the Planet Management screen looked like before I deleted the Capital City improvement.

The second screen snapshot shows what it looked like after I destroyed the Capital City improvement.

Note that the Colonial Generator improvement has disappeared from the list of available improvements.  However, it is not really gone - it is just invisible!  If you hover over the empty space to the right of the Capital Mainframe improvement the popup for the Colonial Generator appears and you can drag from the empty space to a location to construct it.

Here is the link to a save on Dropbox that you can use to recreate the issue, although you should just be able to start a new game and see it there:

https://www.dropbox.com/scl/fi/dm2npi35ipeji5bk2e6gn/Capitol-Issue-Under-1.71.495432.GC4Sav?rlkey=85lkb9ukxf5ltj3fr1rvbok9q&dl=0