Vanilla slabs cast subtle ambient occlusion onto custom blocks. Slabs do not cast ambient occlusion onto Vanilla blocks.

The occlusion cast occurs on all custom blocks: both those with built-in geometries and those with custom geometries. The occlusion is extremely subtle but can throw off aesthetics in add-ons mimicking Vanilla blocks. The effect may be more noticeable in other lighting conditions.

Demonstration

The 2 attached videos demonstrate this issue in daylight. The demos had to be given as a video, as the effect is subtle. Note the slight changes in colors to the custom blocks (set on red concrete) when the slab is placed. This does not occur with the Vanilla blocks (set on green concrete).

Reproduction

This example is taken from the behavior pack in the attached world for convenience. This world can be opened (in Minecraft Preview) for inspection or unzipped to access the code associated with such.