Closely relates to MCPE-170444.

The bug

If an opaque block is placed on top of a wall or next to it, the top faces of the wall post and the wall extensions that rise up to be flush with the top of the wall post are not culled, nor are the edges of wall extensions that touch adjacent blocks

Note that the bottom faces of walls are in fact culled, which is the correct behaviour.

How to reproduce

1. Place blocks next to walls - test all six sides
2. Enter Spectator mode
3. Inspect

Expected results

Faces which can never be seen would be culled.

Actual results

Only the bottom faces are culled. The top faces and side faces are not.