Water with blocks above will convert to flowing water for no reason

The bug

If there are blocks above water, and these blocks are surrounded by water themselves, the water below them will convert to flowing water.

How to reproduce

1. Build a pool of water (at least three blocks deep)
2. Place a few blocks at the surface (level with the highest layer of water blocks)
3. Switch into spectator mode and enter those blocks, or swim in the pool
   → The water below the blocks is flowing