Are you me? I came to the forum to make the exact same thread asking the exact same question.

I governor-ed two low-priority core worlds in the same turn, after having built orbital Monsatium and orbital Thulium extractors on each respectively. And yeah, it sucks to realise your adjacency bonuses AND mining yields are now hampered by a not-great, un-upgradable tile improvement.

Also, I loaded back to a previous save, just to test whether I could go and delete the orbital improvement BEFORE turning the planet into a core world, just to see if that might be a workaround to the problem - but it isn't, it still deletes the underlying resource when you do that

Hello,
I'll bring this up to the team and try to get an answer before circling back

Following Up:
So the short answer is that it is working as designed.

Longer answer is that when you build an improvement to harvest a resource, it's actually replacing the improvement, that is to say that the resource def is in fact an improvement def, so when you destroy the harvesting improvement it in turn destroys the resource improvement.

Now though it is working as designed, the communication on this could be considered a QOL issue to look into, but would admittedly be considered a bit lower priority on the list of things being worked on. Ill be sure to note it in my report

You recalled correctly