Falling water/lava below water/lava source become new water/lava source. It should not create new water/lava sources.
This can be easily used to grief with lava or water placed at world heigh. To remove one placed water/lava source you have to remove every single source from top to bottom.
Why this is set as duplicate, when this is older bugreport and also contains lava, when MC-126912 is not telling about lava anything and is reported later? Also MC-126912 tells that it creates water source between flowing waters, but It's not the case. If you place water in 1 by 1 tube it will create new water sources below original water source even without flowing water on the sides. Reopen please this and mark MC-126912 as duplicate, because this bugreport describes bug better and is reported earlier.