Why moving a label in QGIS sets the label rotation to 0° and how to avoid that?

Question

I have some line labels that are parallel to the line. When I manually move a label with the "moove label" tool, it sets the rotation of the label back to 0 Degrees. It is not parallel to the line anymore. Why is QGIS doing this and how do I stop this?

I'm using QGIS 3.4.13

Answer 1

Because the rotation is actually being managed by the labeling engine.

When you define the placement by data (moving the label), there is no data for the rotation so it is zero.

But you can define a expression for the rotation (Edit... option of the Data defined, Rotation submenu):

line_interpolate_angle( 
    $geometry,
    line_locate_point(
        $geometry,
        make_point( 
            "auxiliary_storage_labeling_positionx",
            "auxiliary_storage_labeling_positiony"
            )
        )
    ) + 90

What are we doing? Retrieving the angle of the geometry at the nearest point of the label placement. The angle retrieved is from North, so we are adding 90 degrees for the label rotation.

For labels not moved, the rotation will be managed by the engine as before.