I believe YouTube uses automated systems to detect when music in its
videos matches music in some database(s) of presumably-copyrighted
music.  (Remember that even if a composition is out of [composer]
copyright, a performance of may still be under [performance]
copyright).

Perhaps YouTube has incorrectly matched your video to some recording of
another piece.  Is there, I wonder, some system by which you can answer
YouTube's warning?

For the record, I don't think "50 years after the death of the composer
and/or performer" is right in America, although it may be in the UK.
There have, I believe, been changes to our copyright law executed after
heavy lobbying from money-grubbers.  Somebody else is surely more
qualified to answer that element of your question.  As far as I'm
concerned, purchasing a music roll does give one performance rights.
Whether the copyright office agrees is another question, I guess!

TJ Fisher