To add a bit of information to this problem or bug:
A label with a multi-line attributed string was placed using IB while View As set to an iPhone 8.
(Making a width of 343 with 16 point leading and trailing constraints to the safe area.)
The attributed text using center alignment and the label set to 0 lines.
Changing the View As set to an iPad,
the label with is now correct at 736 for the iPad and
the 16 point leading and trailing constraints to the safe area still intact.
The attributed text, however, remains as originally laid out:
appears centered in the first 343 points of the (now) 736 width label.
So, the entire body of the attributed text is left justified to the leading 343 points of the 736 width label,
but remains center justifed within that 343 point width area.
To note, another label, with the same constraints to the leading and trailing to the safe area, properly centers whether it is on the 343 width device or on the 736 width device. The only difference is that it is set to 1 line, not 0 lines.
I point this out, because the label is for some reason causing the word wrapping to occur at 343 points instead of the 736 point width. Once the label was created while View As set to an iPhone 8, it seems like it is stuck with the iPhone 8 safe area width. Changing the attributed text does not seem to change this imaginary early word wrap.
Deleting the label, setting IB View As set to an iPad, does however, allow the very same attributed text to honor the full 736 point width of the label for word wrap!