Feedhub was reading feed items, but Discord was rejecting the posts. The embed builder sent the RSS entry's published string directly as its timestamp, which could be an RFC822 date rather than the ISO8601 value Discord expected.1
That made the retry behaviour unhelpful. The item would come around again on the next run with the same invalid payload.
Check the payload at the receiving end
I compared the timestamp variants against a live webhook. The RFC822 value returned HTTP 400; the ISO8601 version returned 204. Leaving the timestamp out also returned 204.
The fix used feedparser's parsed published date, or its parsed updated date as the fallback, and converted that to ISO8601 for the embed.
The issue was not whether the source feed contained a sensible human-readable date. It was whether the value passed into the next system matched that system's contract.
This was a small change in the feed-to-Discord path, but it explained why reading an item successfully could still produce a post that never appeared. Repeating the request could not repair a field that was invalid every time.