Issue:

All slabs will stack to make a "full block" except wooden slabs that came before 1.16 (all wooden slabs except bamboo slabs, bamboo mosaic slabs, cherry slabs, mangrove slabs, warped slabs and crimson slabs).

Steps to Recreate:

1. Pick up a wooden slab that was introduced before 1.16.
2. Place 2 of them, one on top of another

Observed Results:

There is a space of 0.5 blocks between the slabs.

Expected Results:

The slabs merge together to create a seemingly full wooden plank.

Video: https://drive.google.com/file/d/1TO_ySm_hkgnEu0Zyi9xL9yej3xYW2aoL/view?usp=drivesdk