I changed my Termux setup back to generic (not using 'file) and ... this time it worked! Surely the previous problem must have been that I had custom code which was based on an EMMS older release, and that must have caused a malfunction; in other words, user error. But I took the opportunity to simplify my code while making it compatible, and all issues went away.
So I can't offer you any useful insights other than the usual caveat about users (such as myself) needing to ensure there isn't a 'mixed' installation,.
The upside, though, is that the new emms-mpv is obviously solid, and is working perfectly on my Android tablet termux installation. Volume control, seeking, etc. ... all perfect.