Currently, all tags added by the drag and drop feature are added as deck tags. If I have a stack of cards tagged #!Mana and drag a new card in, it is added as #Mana. This is not ideal and makes the drag-and-drop functionality effectively useless for those of us trying to curate long-term global tags.
I recognize this was a choice, given the consolidation of equivalent global and deck tags when they displayed together, but when all the cards in a given tag are globally tagged, I believe it is illogical to interpret the drag-and-drop action as intending anyting other than the matching global tag.
Just ran into this as well when trying to tag a land base package, hope this gets changed or the option is added to make it favor global tags if one exists with the same name.