I have explicit constraints. I expected consistent behavior and really don't understand why I don't get it.

I've tried two cases:
(1) UIImageView directly embedded in a (sub)-controller's view. Fully bound.
(2) UIScrollView directly embedded in a (sub)-controller's view. Fully bound. UIImageView inside the UIScrollView with all four sides bound to the scrollView and with "equal heights" and "equal widths" constraints set.

Both cases behave the same (test case is iPad Air 2 running 11.4.1.