This may be related to the hitbox issue, mobs see the bridges as full blocks and continuously jump while walking across them.

Additionally, short mobs (spiders) take damage while on these bridges.

Hello,
Suffocation is an easy fix and done in dev. The collision box of mobs means if they are 'on' the top they may jump about - it's a vanilla thing. I have added a couple of methods that may help.
Cheers,
Me.