My weekend became too busy with family matters to take further steps until now, and I have not been back here since then to check/reply-further.

A couple of hours ago however I was finally able to spend time on it again.

Following advice/thoughts I have made a new short test piece, 16 bars with the first 4 being the same start-style, second-4 being a first new style, third-4 another new style, and the last 4 bars going back to the start-style.

And it all worked out entirely as supposed!

Based upon that I have now written-out again the song that was problematic at the end of last week, as a clean-sheet exercise with the same base-style, and also with the same style and tempo changes as the original. Again, it behaved perfectly.

I have also adjusted a few things and reshaped a couple of the musical phrasings, and all has continued completely fine!

It is clear that the original contained something that conflicted, however even on close investigation there seems to be nothing in its creation that I hadn't done possibly hundreds of times over the years.

Nevertheless there is absolutely nothing remiss with the short test piece nor the problem song's replacement form today, so in my case "switching it off and then on again" by building a clean new version has reset whatever was in an error state.

Of course, that doesn't help one jot with discovering where the problem actually lay, however as long as a remedy has worked I can live with that smile

With thanks to all for their advice and suggestions that led there, and if not already tried I hope that the same simple step will take care of it for the other enquirer that I see has been experiencing the same thing.