I did some tests. Blizzard UI scale is disabled when ElvUI scale is enabled. So Elvui supersedes Blizzard, which is also the reason why Elvui removes the Blizzard UI scale setting entirely from the Blizzard Graphics settings dialog now.
DG small window is moves around every time when UI scale is changed by either Blizzard or Elvui. I don’t know if scaling is the real culprit here, but knowing it is one possible candidate. Kaliel’s tracker does not move around after reloads, so whatever DG does different seems incompatible with Elvui.