The culprit seems to be the added support for power chords: In 2.18.2, c1:5 generated a c major chord; now it only generates a power chord fifth. My guess would be that Valentin V.'s chord name cleanup in 78225bc1b386e12dc was the point when this changed.convert-ly exists.
I actually thought I had tried this. Obviously I was too stupid to do it correctly.
Sounds more like issue 4614 to me.
True, thanks. Lukas
