Incorrect flow behaviour for waterlogged stairs

Water inside stairs will still check their surrounding blocks to shape the way they look as if they were an ordinary source block. Despite being visually blocked from every side, the water inside will still react to its surroundings and change its appearance accordingly, as if the stair containing it was not actually blocking it..

Steps to Reproduce:

1. Build the set up shown here:
   

   [media]

2. Place a block next to the top-right-most stair
   → observe result

3. Break it
   → observe result

Observed Behavior:

The water changes it's flow appearance despite not actually being able to flow.

Expected Behavior:

The water would stay in a resting state, because it is contained and cannot flow.

Screenshots/Videos:

Notes:

Relates to: MC-259091 MC-127030 MC-91333 MC-239475