Restarting browser (Firefox 129.0.1 (64-bit)) or refreshing that particular feeds after marking them as Read in Brief 2.7.3, does NOT cause the issue described in title, so I think that it may be caused by the source website, but when i click the feed in Brief, the dates of the posts displayed shown to be from October (now is August) and visiting the post shows "Submitted: 2 years ago". Opening the feed URL in a different browser shows:

<rss version="2.0">
<channel>
<title>anonymized</title>
<link>anonymized</link>
<description></description>
<lastBuildDate>Mon 19 Aug 2024 anonymized +0200</lastBuildDate>
<pubDate>Fri 01 Oct 2021 00:00:00 +0200</pubDate>

So this looks like a bug. I do not know how to quickly reproduce it, i just wait some time, maybe a week.

If you need feed URL, please link me where i can provide it in private.