Marker line Simple marker rotation override not respecting follow line direction setting

[klakar]

Describe the bug
When using data defined overrides to rotate a marker in a marker line, the rotation will not respect the "Rotate marker to follow line direction" option for the marker line.

How to Reproduce

1. Create a marker line style with a marker on the central point, use a large arrow as simple marker, note how the arrow behaves when checking and unchecking the "Rotate marker to follow line direction" checkbox.
   
2. Change the Rotation for the simple marker and again note how the arrow behaves when checking and unchecking the "Rotate marker to follow line direction" checkbox.
   
3. Set the rotation with a data defined override for the marker and yet again note how the arrow behaves when checking and unchecking the "Rotate marker to follow line direction" checkbox.
   

When data defined overrides are used, it doesn't matter how the "Rotate marker to follow line direction" is checked.

This makes it much harder to set a data defined override for the marker rotation in relation to the line direction. If this is required, you will also need to calculate and compensate the rotation for the line segment bearing...

QGIS and OS versions

QGIS version	3.14.15-Pi	QGIS code revision	d5114d2
Compiled against Qt	5.12.8	Running against Qt	5.12.8
Compiled against GDAL/OGR	3.0.4	Running against GDAL/OGR	3.0.4
Compiled against GEOS	3.8.0-CAPI-1.13.1	Running against GEOS	3.8.0-CAPI-1.13.1
Compiled against SQLite	3.31.1	Running against SQLite	3.31.1
PostgreSQL Client Version	12.2 (Ubuntu 12.2-4)	SpatiaLite Version	4.3.0a
QWT Version	6.1.4	QScintilla2 Version	2.11.2
Compiled against PROJ	6.3.1	Running against PROJ	Rel. 6.3.1, February 10th, 2020
OS Version	Pop!_OS 20.04 LTS
Active python plugins	quick_map_services; slyr_community; LmOpenData; layout_loader; DataPlotly; Qgis2threejs; qgis_resource_sharing; qgis2web; MetaSearch; processing; db_manager; latlontools

Additional context

I'm using this to create font markers with text offset on the lines. And I want to rotate labels (markers) 180 degree relative the line direction for lines that show labels upside down. This is not possible if the data defined override for marker rotation do not respect the settings for marker behaviour in the marker line settings.