The stair block model renders a part of the texture it does not need to because it is not visible to the player unless if they are in spectator. This can have a minor performance impact because the GPU has to render part of a texture that does not need to be rendered.

How to reproduce

1. Go into spectator
2. Fly inside the top part of a stair block

This will work with any kind of stair. You have to look at it from the side the actual texture is on because of the way the game culls textures will you are inside of blocks. For tracking purposes this is reproducible on versions prior to 1.16.5. I have attached three pictures, one is while looking at the front of the step and the inside, the second one is what it looks like when you go inside of the step, and the third picture (named 1.8.10) was tested in 1.8.10 because the official Minecraft wiki states that the first version of the game to support spectator mode was 1.8. You can see the same issue is reproducible as well as a similar issue which is no longer present in newer versions of the game.