Not sure if this is a bug but mansions seem to be way, way, way more common now than in previous versions. 

A quick seed explore shows on average 2-6 (about 3 average) mansions inside a -20k to +20k by -15k to +15k area in 1.17 and earlier. After running about 200 seeds I only found more than 8 mansions in that range once.

Running the same parameters in 1.18 I am seeing an average of 20+ mansions. This is 7-10 times as many mansions as in prior versions. I've found several seeds with mansions within 300 blocks of each other, a seed with 9 mansions within 5k blocks of spawn and another within 7k. Seed: -1207387014300890272 (all manually checked)

This is not necessarily horrible as mansions are fun and hard to find and maybe this is intentional but I didn't see anything about it in patch notes as far as I can remember and they are far, far, FAR more plentiful now.