I tested exactly this (in Swift) and it works flawlessly.
What happens when you move the button outside of image ?
The behavior is the same if the button is on the UIImageView or not on it.
I am reporting this as behavior in my Xcode Simulator (tested 5S, 6 and XR). I made Xcode install it on my actual device (iPhone 5S) and it worked flawlessly. As I write this message I am running the simulator again with it displaying beside the browser. After a couple of minutes the button actually goes from GREY to disappearing. This must be a bug in the Xcode simulator because I am running no other applications other than Xcode and Safari and my machine has plenty of RAM (see below for my specs).
I am running MacOS High Sierra Version 10.13.16 (17G4015) with Xcode Version 10.1 (10B61)
on a MacBook Pro 13-inch Late 2011 machine with 16GB or RAM
Thank you for checking on this! I thought I was going crazy.
How can I log this as a bug with Apple?