I have this bridge that I have move in using a script. In-game, zombies cannot go on the bridge and some don't even target the player when they spawn. Does anyone know what's causing this problem with their path?

Try:

"Expand your navmesh to surround your new area."
From: https://ugx-mods.com/forum/index.php?topic=12978.0

Sounds like a navmesh problem.
Try finding a tutorial on zombie navigation or something.