Solidworks will never mirror Hole-Thread-Data, reason being is that a Right-Hand-Thread would then become a Left-Hand-Thread in the Mirrored Part.
What I have found, is that when I am in the mirrored part, I select the hole feature face, to highlight it, then right click, select edit feature, and then a "Transfer" checklist pops up over the browser tree. I check the "Custom Properties" check box, and then check OK above. I believe that the hole features are then carried over to the mirrored part and will be noted properly in the drawing.
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