I am running the Java version of Minecraft currently set at the latest snapshot 18w16a. When creating a world using the new World Type: Buffet option and selecting any of the ocean options a stronghold will not spawn. Some examples are Ocean, Deep Ocean, Warm Ocean, Deep Frozen Ocean, etc.
I can use /locate and it will give me the coordinates for the nearest stronghold but I cannot /tp to it and when I arrive at the location there is nothing there (just normal stone/gravel and maybe a cave). I have also used Eyes of Ender to find the spawn location of the structure but when I get there, nothing is under the ocean floor and the Eyes continue to point downward until I hit bedrock.
In non-ocean-based Buffet style worlds, the strongholds seem to generate fine. Also, in normal world generation done in 18w16a the strongholds seem to have no problem with spawning underwater in ocean biomes.
Mineshafts are unaffected and spawn fine in all Buffet ocean zones under the ocean floor.