Sometimes the world generation doesn't generate flowing water where it should. An easy to see example is using this seed: 8031370935278107419

Go to 41/710/171 and take a look - broken water block on the side of the floating islands that now performs wonderfully well.

I just went to the seed/coords and see this, all of the water blocks are flowing as normal.

I still see this regularly but lately I'm wondering if isn't worldgen lag. Sometimes the world gen completes and then suddenly water/lava start flowing.