The issue has been found
…/wp-content/themes/colorway/style.css?ver=5.5.3
.animated{opacity: 0}
The question I have is why that code suddenly appeared, or why it suddenly started taking effect, and why does it only cause a problem in Desktop view not Mobile?
My guess is that the theme was auto-updated and that something didn’t jive. Years ago I modified the CSS in a couple of files to lower the animation delay. Curiously, as mentioned, at least one other person had the problem years ago so… I dunno.