Hmm, I may be doing something wrong here then.
It seems to me that when I export a flashcard list, the definitions don't go with it. If, for example, I export a list, delete the original set from within Pleco and then "re-import", the defintions that get associated with the characters in the imported list depend on which dictionary is currently being displayed in Pleco. If the User dictionary is displayed, then, for the most part, there won't be a problem because the correct definitions will be filled in on the imported flashcard list. (Even in this case, though, if there are multiple user-created definitions for the same term, it seems that Pleco will just automatically choose the first-made user-created definition for that term). If the dictionary is set to Oxford or ABC, Pleco will search that dictionary first and fill in defintions for the imported list from that dictionary, then look in other dictionaries if it can't find the particular term...
Anyway, this is what I think I experienced. Any confirmation of whether this is how it works would be appreciated.