Most probably user error, so could someone point out or confirm similar bug.

Drop a clip in source record monitor, mark an in point only. (for testing, easiest at the start)
Move the play cursor (blue line) in source window to wherever, but not the end.
Place clip on timeline using splice/overwrite.
In previous versions this would put that entire clip on the timeline as the in point was at the start and there was no out point set, whereas 2018.7 is creating a hidden out point based on where that blue line cursor was left.
So in my instance, instead of an 4 min clip going on the timeline, a 34sec clip has.
Deleted the usual mcstate, site settings, diff user etc makes no difference.
Its not doing in it 2018.4 which we've put back on our edit 1

Have managed to get to the bottom of it.

At looks of it, upon upgrade for 2018.4 to .7 for existing avid users on our suites it has enabled single mark editing in composer settings.
Turning that option off, everything works as it should.

Creating a new 2018.7 user has the option disabled by default, so must be a glitch in upgrade somewhere.