Linked files are embedded on save

Picture this: I drag and drop a linked file into an .ai file. For the remainder of the session, it’s listed as “linked file” and behaves accordingly. But when I save, close, and reopen the file, it’s embedded.

This is making the .ai file impractically large (to the point that Illustrator consistently freezes when it finishes saving). I don’t know what I’ve done to make this happen; it wasn’t always like this and other .ai files don’t have this problem. What have I done and how can I undo it? Thanks.