To reproduce, do the following:
Press PrintScreen to start capping a screenshot
Hold shift while moving the cursor to lock either the horizontal or vertical movement axis
Move the cursor in the direction that should now be locked
Once you have your chosen starting pixel "selected" under the crosshairs of the magnifier, ensure that the mouse was not still in the perfect spot in the locked direction (see previous step) and start dragging for the screenshot
Finish taking the screenshot
Expected behaviour: the screenshot is taken starting at the center position of the crosshairs of the magnifier at the time when dragging started.
Actual behaviour: the screenshot is taking starting at the pixel your cursor happened to be hovering over when you started dragging.
Impact: this bug makes the "shift-to-lock-axis" functionality useless for selecting the starting pixel of the capping process. The same bug does not appear to affect the second/final point of the screenshot.
Windows 10 build 1803