Describe the bug
The Buffs Module seems to be causing a taint when exiting edit mode. It seems to be related to the focus frame, and or Objective Tracker, however I'm not an expert so may be wrong. I commented out all modules in the load.xml to track down the module that was causing the taint & it was the buffs' module causing the issue. I have pasted the taint log below, no add-ons except SUI were active at the time.

2/10 22:02:00.837 Global variable OBJECTIVE_TRACKER_UPDATE_REASON tainted by SUI - Interface/AddOns/Blizzard_ObjectiveTracker/Blizzard_ObjectiveTracker.lua:1416 ObjectiveTracker_Update()
2/10 22:02:00.837 Interface/FrameXML/EditModeSystemTemplates.lua:1819 ObjectiveTrackerFrame:ApplySystemAnchor()
2/10 22:02:00.837 Interface/FrameXML/EditModeSystemTemplates.lua:270 ObjectiveTrackerFrame:UpdateSystem()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:1039 EditModeManagerFrame:UpdateSystem()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:1027
2/10 22:02:00.837 secureexecuterange()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:1029 EditModeManagerFrame:UpdateSystems()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:899 EditModeManagerFrame:UpdateLayoutInfo()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:1179 EditModeManagerFrame:RevertAllChanges()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:186 EditModeManagerFrame:ExitEditMode()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:208
2/10 22:02:00.837 EditModeManagerFrame:Hide()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2899 Frame:SetUIPanel()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2930 Frame:MoveUIPanel()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2961 Frame:HideUIPanel()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2627
2/10 22:02:00.837 Frame:SetAttribute()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:3245 HideUIPanel()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:121 onCloseCallback()
2/10 22:02:00.837 Interface/SharedXML/SharedUIPanelTemplates.lua:149
2/10 22:02:00.837 Execution tainted by SUI while reading OBJECTIVE_TRACKER_UPDATE_REASON - Interface/AddOns/Blizzard_ObjectiveTracker/Blizzard_ObjectiveTracker.lua:1439 ObjectiveTracker_Update()
2/10 22:02:00.837 Interface/FrameXML/EditModeSystemTemplates.lua:1819 ObjectiveTrackerFrame:ApplySystemAnchor()
2/10 22:02:00.837 Interface/FrameXML/EditModeSystemTemplates.lua:270 ObjectiveTrackerFrame:UpdateSystem()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:1039 EditModeManagerFrame:UpdateSystem()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:1027
2/10 22:02:00.837 secureexecuterange()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:1029 EditModeManagerFrame:UpdateSystems()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:899 EditModeManagerFrame:UpdateLayoutInfo()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:1179 EditModeManagerFrame:RevertAllChanges()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:186 EditModeManagerFrame:ExitEditMode()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:208
2/10 22:02:00.837 EditModeManagerFrame:Hide()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2899 Frame:SetUIPanel()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2930 Frame:MoveUIPanel()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2961 Frame:HideUIPanel()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2627
2/10 22:02:00.837 Frame:SetAttribute()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:3245 HideUIPanel()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:121 onCloseCallback()
2/10 22:02:00.837 Interface/SharedXML/SharedUIPanelTemplates.lua:149
2/10 22:02:00.837 CloseDropDownMenus()
2/10 22:02:00.837 An action was blocked because of taint from SUI - ClearFocus()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:1560 Frame:ResetTargetAndFocus()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:1536 Frame:OnEditModeExit()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:188 EditModeManagerFrame:ExitEditMode()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:208
2/10 22:02:00.837 EditModeManagerFrame:Hide()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2899 Frame:SetUIPanel()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2930 Frame:MoveUIPanel()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2961 Frame:HideUIPanel()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:2627
2/10 22:02:00.837 Frame:SetAttribute()
2/10 22:02:00.837 Interface/FrameXML/UIParent.lua:3245 HideUIPanel()
2/10 22:02:00.837 Interface/FrameXML/EditModeManager.lua:121 onCloseCallback()
2/10 22:02:00.837 Interface/SharedXML/SharedUIPanelTemplates.lua:149

Bug Seems to have disappeared, since reset.