OK - I see what it is...
I used to select a range on the timeline, with *nothing* selected - and I could then go select a track (or tracks).
Now when you select a range on the timeline, with *nothing* selected - it auotmatically selects *all* tracks (which is never what I want) ;-)
So, all I have to do is select a track first, and *then* select a range on the timeline - and it selects that range, for only the selected track.
Problem solved, by clicking in a slightly different order ;-)
I *knew* it wouldn't be complicated to figure this out - I just never bothered to do it!
So, now we all know ;-)