Note: The problem appears to have been resolved. But more details because I failed to give them:
I completely failed to mention I'm running CloudLinux+CageFS, I'm sorry. I keep forgetting not everyone's using it. d'oh
A while ago¹ I set the default PHP 7.2 options to long execution times and large upload limits just because that works well on my server. Most definitely since that time, it's worked because my main client keeps running into Wordpress upload limits because I keep forgetting to set new domains to use PHP 7.2; but doing so solves the problem.
What's weird about this is that about a week or so ago when she noticed the problem, I tested on other Wordpress sites and the problem appeared to be on all Wordpress sites using Divi — so she opened a ticket with Divi, who solved it by putting a php.ini with just the two lines from my first post inside the public root of one of the sites.
I definitely haven't added php.ini, local.ini, or user.ini to the document root anywhere.
I just tested again on a site that I haven't touched and… it works again. So who knows?
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¹ Like 2-3 months ago, if not more