I built a mob farm in the ocean biome (above sea level), probably lukewarm ocean or something. It worked greatly but no zombies managed to spawn anyway. Creepers, skeletons, spiders and even witches ARE spawning. I have also observed zombie on the beach, so I'll assume there is no problem with my world.
Related issues
duplicates
Comments
migrated
migrated
There is known issue for zombie and pillager having trouble spawning from cave spawn mechanic. There is a fix in 1.13 changlog. Also there is a ticket already for this kind of problem relating zombie spawn.
The solution as of now would be coated the roof with slabs will allow zombies spawn on the spawning platform under the roof.
I have the same problem. Did you find a solution?