This is a minor bug, but I've seen it many times. When my influence is rising in a region that already has rival asteroid bases, I keep getting incorrect notification messages like this one:

"<starname> Mining Base 1 has defected to the <other racename>."

The notification is incorrect. The mining base already belongs to the other race, and has all along. My guess is that some start-of-turn calculation of the current influence in the asteroid mine tile is raising a false alarm. At no time during a turn does the asteroid mine ever switch allegiance to my race.

I'll raise a support ticket and attach screenshots and my savegame.