Water flowing through waterlogged stair corner

1. To reproduce the bug, place two dissimiliar stairs to form a corner.
2.  Waterlog the corner.

What happens here is that water will flow from what is supposedly the 'wall-side' of the cornered stairs, but only if the two stairs are of different types.

In my build I used copper, which oxidizes, hence why the problem did not initially occur.

 

Thank you in advance.