Unclear when lines (line layer) show up on the map

[beunderf]

Our line solution is not showing all line items all the time (the red dots are the line layer items, only zoomed out they appear as dots)

When we move the map, it is changing which line items it is showing on the map. What is causing this behavior? 

Situation 1:

Situation 2, same selection, same zoom level, only dragged the map a little to the right)

Who can tell us what is happening here?

Frank