The bug

A player can only climb on a trapdoor if there is a ladder below. This does not work with any other climbable blocks which is inconsistent.

This is not related to the collision box of the ladder, see "Code analysis".

How to reproduce

1. Place any climbable block other than ladders (e.g. vines) below a trapdoor
   Note: Blocks attached to the side, like vines, must be attached on the same side as the trapdoor
2. Open the trapdoor and try to climb through it
   You cannot climb through the trapdoor

Code analysis

20w10a, Mojang names
net.minecraft.world.entity.LivingEntity.trapdoorUsableAsLadder(BlockPos, BlockState) explicitly only checks if there is a ladder below the trapdoor and if that is the case treats the trapdoor as climbable.