Drowned have an AI bug in the current snapshot that prevents them from exiting a body of water is below a certain y level. (For regular drowned, this occurs if the highest water block is at y=61 or below, and it happens for baby drowned at y=60 or below.)
Video of the bug: https://www.youtube.com/watch?v=UHeeXuB9AJM
EDIT: thanks to the new drowned swimming animation, we can see that they get stuck in their swimming AI and are mostly unable to transition to their walking AI, which is necessary to leave the water. They can sometimes randomly transition to the walking AI after longer periods of time.
Can confirm for 19w37a