I think I realized the problem. Part of my Writeas workflow has always involved using scheduled posts as a method of previewing how a post is going to look on the site as I’m composing. In the past (namely, in the post I was referring to originally in this thread and others,) I had used the same post as the final, published one, by changing the date from some point in the distant future to the current time.
It didn’t occur to me until just recently that this could be very confusing for the site’s backend, so I’ve been making an effort to create a new post at the point I’m ready to publish and deleting the scheduled draft. Since, I’ve had absolutely zero issues with new posts arriving in my inbox.
I thought I’d update this thread just in case anyone was doing something similar.