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Sketch changed when mirrored

I've versioned a documet that I'm having a problem with: https://cad.onshape.com/documents/d00ceaba15e3ae5795e20cc8/v/e1306259ed2b791bba55a81d/e/5b289bdfe74c787a08ec07e3
There's a sketch "Sketch 2", on a plane parallel to the "Front plane"
The angle between two lines of this sketch is as follows:

Then I've mirrored this sketch by the parallel mirror plane "Front"
And on the mirrored entity the angle between those lines is different:

How could it happen?
I've checked that all the vertical and horizontal lines on both sketches are parallel, but not the inclined lines.
There's a sketch "Sketch 2", on a plane parallel to the "Front plane"
The angle between two lines of this sketch is as follows:

Then I've mirrored this sketch by the parallel mirror plane "Front"
And on the mirrored entity the angle between those lines is different:

How could it happen?
I've checked that all the vertical and horizontal lines on both sketches are parallel, but not the inclined lines.
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Best Answer
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NeilCooke Posts: 1,886
That's very unusual. The construction line at the bottom is shorter and I can't see the reason for it, so it may well be a bug. Please open a support ticket (help > report a bug). As a workaround, create a new sketch on a plane where the mirror should be and "use" the geometry from the first sketch.Neil Cooke, Director of Technical Marketing, Onshape Inc.5
Answers
Would it be to do with dropping external references?
https://forum.onshape.com/discussion/8603/option-to-not-to-drop-external-sketch-references-in-feature-pattern
EDIT: I take that back. Deleting the external constraint in the lower left corner fixes the mirror. The link you shared refers to a feature pattern limitation, whereas this presents itself as more of a bug, but it's the same thing. I will add this as a test case for fixing.