The bug

The way in which the oak trapdoor texture is shaded, alongside the colors used, appears very different from the oak door:

• The oak trapdoor has a dark ring around its outside whereas the oak door has an alternating light/dark frame to produce a directional lighting effect.
• The oak trapdoor's shading is noisy and appears to have hints of green and purple, whereas the oak door is a more plain brown.

How to reproduce

1. Place an oak door
2. Place an oak trapdoor
3. Compare the textures and art style
4. Repeat with another wood type, such as birch

Expected results

Oak doors and trapdoors would appear very similar in art style, palette, shading and so on.

Actual results

The discrepancies noted above can be seen.

Further notes

MC-165503 may exaggerate the perceived effects of this issue - it may be advisable to use a custom resource pack to fix it before comparison, or perform the test on barriers or another block which does not impart ambient occlusion.