Solved: The screen/display was replaced by the store where it was bought. No issues anymore and also no more brightness flickering when using Freesync in games.
Problem: My Samsung C24FG73 shows horizontal lines / artifacts when running at 144hz over Displayport. It appears on startup and gradually disappears over time. The amount of lines increases with colder room temperature and takes an increased amount of time to disappear.
No problem when running 120hz over Displayport or HDMI.
Has been RMA'ed and waiting for reply (august 2019).
