The sharper a route turn and the closer the next waypoint is located to the turning point, the higher the probability that the aircraft is getting stuck in a flight path loop.
Can this also happen when the aircraft is flying in the background with AP+ or does the aircraft strictly follow the route line in that case?
This can happen even under AP+.
Ok, got it! Thanks for the ultra fast response! 
Lnav currently works on transitions based on 1nm rule which can overshot waypoints if there is another waypoint next less than 4nm on the angled path and if the gs increases or rather high that will make the aircraft unstable which can loop you around the first transition.