If you have any roads, bridges, incomplete areas, etc that might have a low compactness score, you could have lost some prestige there - but it should be temporary. If you correct it, it should go back up.
Since your issue happened after the last update, I’ll suspect there is an area with a low compactness score in your area. (if you’re sure no one removed beacons/blocks, no bridging settlements, etc)
If this is your area/town on that planet, there is a large area/beacon that has a low compactness score and will need to be filled in more to correct it. (yellow area = low compactness)
