These marks appear as a two adjacent pixels with dejavu sans at sizes 9-10, which looks weird and often confusing. It makes them look like one dot or a short horizontal line. They should be separate. This also happens with sans mono, but only at size 10 and the "ä" appears fine with separate dots (though one sided).  The best would be if the two dots would appear two pixels apart in these occasions, because one pixel distance would put them closer to one side and therefore asymmetric which doesn't look nice. I attached an image showing the problem.
Tested versions: 2.31, 2.32. Only tested on windows, so I don't know if its like this on other platforms too.