This is being caused by this bug .
As I understand it the game stores mansion details in a .dat file separate to where chunks and the rest of the terrain are stored. Resetting a chunk with a mansion in without deleting the data on that mansion in the mansion.dat file causes the mansion not to generate again. When the chunk does get generated again the game finds the mansion data in mansion.dat and thinks it's already been generated so it doesn't do it again. Which leave us with no mansion.
Apparently is also the cause of mansions sometimes only partly spawning of being cut in half.
@CraftyMyner Seems like you can fix this by resetting the mansion chunks again and deleting the mansions in reset chunks from the mansion.dat file.
This is presumably going to cause other structures such as ocean monuments and villages to fail to generate too. I seem to remember a post about a missing monument and one about a missing village. I assume these could be fixed in a similar way.