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Here is another situation with trying to chamfer/fillet 2 polylines.
It's very strange. If I tryed join this, by click on point 1 and 2 - it's 2 polylines (PL) - it join this PL not correct = one extendeted / trimmed PL.
If I explode PL with point 1 to line and try chamfer line (1) with PL (2) - it works fine, but it begin one new (right) chamfered PL and one (original) line.
And at the end, if I copy this 2 PL to new empty DWG it works fine.