When orbiting objects in Prepare and Preview mode (orthogonal view), the Z axis is locked in a vertical orientation. Now matter how you orbit the objects, Z axis always points straight up.
But this changes when Seam Painting mode is active, orbiting view becomes tumbling view which Z axis can be freely move in any orientation. This makes vertical option not useful for painting vertical edge.
After exit Seam Painting mode, the orbiting view is showing in the orientation in tumbling view. It will locks itself back to orbiting view as soon as you start orbit the object.
This makes painting straight seams of complex shape a impossible task. A workaround now is to switch back and forth between perspective and orthogonal view. By doing so, the Z axis will correct itself.
Take a look how Orca Slicer does it. Hopefully this gets fixed in the next update.