It appears that the X coordinate on the "XYZ" display of the F3 menu is either incorrectly rounded or offset by 1. The "Targeted Block" display seems to be displaying it correctly however, evident by the command block screenshot I've included.

Between the 2 screenshots of the F3 menu, you can see there is a desync of the X coordinate between the "XYZ" and "Targeted Block" display.

One screenshot has been taken in 1.16.5, and the other has been taken in 21w06a. In 1.16.5's screenshot, the display has no desync. Meanwhile, 21w06a's screenshot shows the desync.