So on a server I play on, we have bingo cards to catch specific Pokemon. A group of players had been searching for Drilbur for hours at this point. I went onto a single player world and found that no cave system no matter the y level was would spawn a single Pokemon. In some instances, with caves close to the surface, only Pokemon specific to that biome (such as joltik) could be found in caves.

Hello, and thank you for the report! I've confirmed the issue and moved the info to our internal tracker. Have a nice day!