I noticed since 7.1 that if you open the world map and open any dropdown in the UI (from the world map frame or any other frame) the dropdown will suddenly close itself.
After doing some digging I found that it happens every time WORLD_MAP_UPDATE is fired (every few seconds, for world quest stuff I guess). I have traced down the issue to the WorldMapLevelDropDown_Update() function being called by WorldMapFrame_UpdateMap(). I have not gone farther but something inside this function makes any opened dropdown closed (maybe UIDropDownMenu_Initialize() ?).
I have an add-on that adds options to the map filter dropdown menu and I got users reporting me this bug so I ended up implementing a custom fix that just check if DropDownList1 is visible before calling WorldMapLevelDropDown_Update().
local oldWorldMapLevelDropDown_Update = WorldMapLevelDropDown_Update
if not DropDownList1:IsVisible() then