After a bit of experimentation, I found out that the animation happens when I assign the UICalendarView's selectionBehavior property to an instance of UICalendarSelectionMultiDate (to enable multiple date selection) just before animating the changes to the UIStackView's contents. I'm not sure why that triggers an animation but I was at least able to fix my specific issue by simply assigning the new selectionBehavior in the completion block of my animation.

Thank you for the reply. I will have the user go through these steps and hopefully he will get it resolved. I do have to say it is a bit unsettling that this condition is possible and, even more so, that it is not an error condition at the code level. I hope someone looks into it because I'd hate to have another user run into the same trouble. Thanks again for the help!

UPDATE: My original post was misleading as I was misunderstanding part of the problem specific to my application. In any case, there is still a problem and it turns out it appears to be an unexpected change in WKWebView's print rendering (not html rendering as I originally thought). All of a sudden printing background colors seems to be off by default. I was able to resolve the issue using this answer on SO: https://stackoverflow.com/a/75882507/1165843.