The minecraft:placement_filter block component dictates where a block can be placed. In Vanilla, placement checks using such are performed against a block’s resolved placement. In this component, placements are only allowed when directly targeting whitelisted faces.

Demonstration

The following video shows this bug in action:

Note that the Vanilla block is placeable on the sides of adjacent blocks. However, the dandelion’s actual placement filter only supports grass-like top surfaces. Placement attempts are tried on the sides of adjacent blocks for the custom mimic, but placement does not occur until directly targeting the associated face.

Reproduction

This example is taken from the behavior pack in the following world attached for convenience:

This world can be opened (in Minecraft Preview) for inspection or unzipped to access the code associated with such. Placements should be attempted to verify the bug as shown in the video.