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The .dtl / drawing property witness_line_offset has no effect on tangent leaders of feature control frames.
Dragging the handles of tangent leaders of feature control frames has no effect. (Dragging the handles of leaders of datum tags works.)
It's a feature, not a bug.
Work-around is to attach the tangent leader to a location WELL INSIDE the geometry. Add a break from INSIDE the geometry to a point outside the geometry.
Thanks to Frank for the work-around and sketch.