Feed repeatedly lists old items as new

[Kissaki]

For many weeks now, the RSS feed repeatedly reports a bunch of old items as new, presumably when a new post is published.

Some exploration, context, references:

PR #1780 fixed an issue early last year regarding the filename inferred date sorting.

What changed?

Looking into recent config changes, #2075 dropped plugin references from the config, presumably fixing redundant declarations. Could that be the issue, causing the plugin configuration from the site rather than theme no longer being applied?

[fdncred]

I don't get any old items as new in feedly.

[Kissaki]

Feedbro

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The feed has a pubDate and a lastBuildDate. Items have a guid (URL), but no date.

Does your feed reader not show updated items previously marked as read, by any chance?

[fdncred]

Feedly has a default filter of Unread. After you've read it, it doesn't come back as unread.