This happens if you set _dataRepeater.ItemHeaderVisible = false
. Since item headers are not visible, the repeater shows this dotted line in order to show which is the current item of the repeater.
Turns out there is actually a simple way to achieve this:
_dataRepeater.ItemHeaderVisible = true;
_dataRepeater.ItemHeaderSize = 0;
Which will achieve the same but without the dotted line.