i did more of my own testing, and it seems that the ancient cities are actually generating properly under the world, but the deep dark biome isn't.
i am getting the same when i upgrade my old 1.17.1 world to 1.19. neither the deep dark or ancient cities are generating beneath new chunks, despite dripstone caves and lush caves still generating underneath.
can confirm in release 1.21, but its seem that the issue with frame drops have become less severe in 1.21. My game can handle about 4x more double sided, dyed, and glowing signs than it could before in 1.20.