Just to clarify further - my conclusions are that ...
(1) When I select any section of audio by dragging
in the timeline (my selection won't snap to zero) and if I then drag and move that selection, it will result in new clips with resultant non zero crossings. (what I was experiencing)
(2) However, when I select a section
within the track, the snap to zero crossing will alter my selection to the nearest zero and act as expected.
(3) Snap to zero audio crossings does not work within
rex or groove files (what I was experiencing). They need to be rendered first.
If this is the intended behaviours, I can live with that, and thanks to those who contributed above.