Water created on top of newly created water layers will not spread. Create an area and fill to sides with water to create an infinite water source filling the entire area. Next, attempt to create a second layer of water on top. The water will not spread as intended and instead simply fills the single block where you placed the water. Tested on both single and multiplayer.

Rebooting the server or saving and reopening a single player would does not seem to fix this. However, this problem existed yesterday for 1.13 pre1. Today we updated our SMP server to pre2. After updating I got on and was able to spread the previously broken layer but could then not add another layer on top. Not sure what refreshed the water layer during the update.