Noticed this a month ago, created account to report this.

When boxed between two walls or two more vertical trapdoors, vertical trapdoors stop mobs from jumping or moving (or confuses phantoms-needs two trapdoors wide instead).

Problems that arise from this:

-hostile mobs (zombie and wolf and) will not chase target (wolves are even unable to attack)

-mobs cannot flee: skeleton and creeper from wolf and ocelot

-snow golem will aim for mobs, but snow ball will shoot perfectly along an x or z axis(basically following the row of trapdoors)

skeletons and pillagers can still shoot, however.
will check how it affects endermen, horses and spiders out of curiosity

May be connected to MCPE-45560 and MCPE-13451