Likely caused by the same code as MC-172449, but posting a new issue since 1.19 exposes a new behaviour of that issue that didn't exist prior.

Upgrading worlds from 1.17 or below directly to 1.19, or even if you upgraded it to 1.18, having some old 1.17 chunks never loaded in 1.18 and thus never generating those terrain below y=0, causes new Deep Dark biomes to "generate" in said old chunks. Going down to where the Deep Dark would have generated, /locate biome tells you you are 0 blocks away from the Deep Dark which is not true because the F3 Debug Menu says otherwise (Hence MC-172449) and no biome decoration is generated. However, the structures in Deep Darks somehow still generate, leading to Ancient Cities outside of Deep Darks and also with very little sculk, since only the sculk that comes with the structure are placed. This is not the case with Lush or Dripstone caves, as one of those cave biomes below Y=0 properly generates when upgrading 1.17 worlds to 1.18-1.19. F3 correctly indicates that it is one of those cave biomes, and biome decorations are placed.

This bug is what happened to Etho in his first video of him playing around with 1.19. He found an Ancient City with very little sculk. https://youtu.be/u4CnKLIGfnI (Go to 26:57 in the video when he finds the city)

To reproduce, find any seed online that preferably have a Deep Dark near spawn. Generate the world in 1.17, make sure to load the chunks where the biome should be, then upgrade the world to 1.19. Notice the sculk-less city.