Unless I'm missing something here, the behavior is not consistent.
According to
https://getmusicbee.com/forum/?action=search2<element id="Content[AlbumAndTracks].GridTrackSplitterLine" bg="xxx,xxx,xxx" />
the track splitter line is only used if there are no alternating row colours set (or the same)
From trial and error, I found that the splitter lines do still appear even if there are alternating row colors set. (and that they render using the color set for the alternating row)
But if you comment out the corresponding splitter line element, then they aren't rendered.
For tracks that means commenting out:
"Content[TrackDetail].ItemSeparatorLine"
For Now playing tracks it's
"NowPlayingList[TrackDetail].ItemSeparatorLine"
And for Album and Tracks it's
"Content[AlbumAndTracks].ItemSeparatorLine"
Except for Compact Grid view where they render even if you comment out
"Content[AlbumAndTracks].GridTrackSplitterLine"
Surely this isn't intended behavior.
And the problem affects even the most polished skins, with for example Centrol being affected:
