I've worked on this problem for some time on a personal project, and I'm pretty convinced you can basically solve this problem without deep learning or AI techniques, and instead use non-negative matrix factorization[0] as a bank of note templates (from their spectrograms). I have a fairly well working proof of concept and the approach is supported by the literature.
[0]: https://en.wikipedia.org/wiki/Non-negative_matrix_factorizat...
_edit_
That said, you'd probably need something more hard-core for extraction from an actual track, so you're probably right.