Background lines displayed in lower resolution

Something strange started to happen today and I can’t find a reason. Background lines are displayed in lower resolution than foreground lines (see images). Any idea on why this may be happening?

Sometimes it looks like this when the same path has been pasted over itself a couple of times. The anti-aliasing will not work properly then, and it looks like lower resolution. Can you check if you have duplicate paths?

That was the first thing I checked but there were no overlapped contours in the background (or foreground). Yesterday I think at some point the issue disappeared (I don’t know how or why). I will continue checking as soon as I come back to Glyphs (today I must work on other editors).