Curious why holding ctrl to drag and drop text would be necessary (or recommended). Are you dragging and dropping or copy/pasting? I admit drag 'n' drop text editing is not my cup of tea. I disable all code-completion, code-collapsing, and drag and drop wherever possible in my Code Editors. In my experience, their only purpose seems to be to do things that I don't want to be done.

Does ctrl leave the text in place while dragging a copy to a new location?
Otherwise, Turtle91's explanation for what's happening seems highly likely. Holding ctrl while left clicking a class name has had that behavior for quite a while now, I believe.
If that's indeed the case, overlap with a Sigil feature is one more reason why drag 'n' drop in Code View is unlikely to be officially supported.