The Bug
When having a trapdoor up (so it can support a falling block) with a gravity based block on it then closing a trapdoor so it's on the ground doesn't have the gravity based block pop off despite no longer being a supporting block.

Replicate

• Have a trapdoor up
• Have a gravity block on top of the trapdoor
• click the trapdoor so it's on the ground
• Notice the block doesn't pop off