This is the same issue as MC-114779.

The bug

The uppermost cuboid of stairs has a bottom face which can never be seen under normal circumstances. Rendering it is pointless.

This face was removed from Java Edition in 2019 (see MC-114779).

How to reproduce

1. Place a stairs block
2. Enter Spectator mode
3. Fly inside the stair block

Expected results

The bottom plane of the upper cuboid of the stairs would not exist.

Actual results

It does.

How to fix

Remove this face, since it isn't needed.

It's actually possible to remove two faces and still get a visually identical result - this solution is discussed in MC-262460 and is preferable.