Why does the Gantt chart start 18 months back?

(Using OP 3.14.3)

I use OP to do some slightly longer-term (months-to-quarters) planning and have a single rolling document I use for this, deleting tasks/task groups once completed and more than a few weeks in the past. None of the current tasks or task groups I have in the document have scheduled or constrained start dates more than a few weeks back (as confirmed by both examination of all tasks in the inspector and the “ghost” left square brackets in the timeline). However when I open the document it always defaults to a view showing me a lot of white space to the left of the earliest tasks in my actual time line, stretching back over 18 months. It’s almost as if there are “ghost memories” of old, long-deleted tasks some where in the document.

Minor irritation but it is annoying me! Can anyone suggest what might be causing this behaviour?