I believe that is happening because wc3 only supports rotating a pathing map 90 degrees. You can see an example here:
Not sure if this is already a thing or not I am making custom farm structure. I have already got the model settup and working (for the most part... had to kludge it by making the crops a seperate model (and having a trigger place it for me when u create main structure, and a trigger destroy it...
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Even if the model is visually rotating to the angle you specified, the pathing map "snaps" to the nearest "supported" orientation. For your bridge, the supported orientations are vertical and horizontal:
That's why Blizz made variations for each bridge orientation (vertical, horizontal, diagonal 1, diagonal 2). In fact, the vertical and horizontal one use the same underlying pathing texture, it's just that one of them has a different fixed rotation set in the object editor (same for diagonal 1 and diagonal 2).
So in your case, I'd suggest making a "diagonal" version of your bridge as well. Depending on the angle between the caster and the target point, you'll want to create either a "vertical" bridge (with a rotation of either 0 or 90 for a horizontal bridge) or a "diagonal" bridge (again, with a rotation of either 0 or 90 depending on which way the bridge should go). Instead of allowing any arbitrary angle, I'd recommend just "snapping" the bridge rotation to those fixed angles (0, 45, 90, etc.).
The main reason Blizz probably did this was because the pathing for most "static" objects (destructables, bridges, buildings, etc.) are defined by a bunch of small 32x32 squares--and you can't easily rotate that pathing map to any arbitrary angle and get it to work accurately without sacrificing some squares. So they likely settled on what they have today. But if you wanted to support more angles, you could always make your own custom pathing maps for those orientations--and then make a separate object editor object for that too.
