Podcast Feed Limited by Feedburner
The last two episodes of Developer Tea haven't shown any downloads, so naturally I reached out to Simplecast to get an answer.
While waiting, I kept trying to diagnose the feed directly, having totally forgotten that the RSS feed itself was being managed by Feedburner.
Unfortunately, Feedburner has a limit to the size of feed it will parse - specifically 1024K.
Luckily, Apple has made managing podcasts easy; updating an RSS feed is as simple as entering it into a field in the Podcasts Connect interface. Goodbye, Feedburner.