Pretty much like the Summary says, taking water from the ocean with a bucket leaves an empty block instead of it being filled like it did previously. I did verify that a 2x2x1 area of water does not exhibit the same behavior (i.e. infinite water sources still work).
Sorry, it looks like this duplicates MC-128690. I was only looking for 18w19b reports when I created this. I can confirm it's still an issue in 18w19b.