An update to this thread (as that one is locked): Sometimes too many navigation arrows are shown
I contacted Garmin Support, and after lengthy discussions and sharing lots of screenshots of what obviously is a bug/shortcoming in their software, they eventually told me that they are not going to fix this as 'it's not a defect of the device', and that as a workaround I should not create routes that have so many overlapping roads... :-(
Very dissapointing from Garmin...