Sorry for the delay, it took some time to collect more data on this issue.
I don't think it's a issue with the feeds or messages, as almost all of my feeds are affected and messages formatting seem fine - dates are correct (i've collected some for the team to look at). It seems to me there is a problem with messages database, which is not consistent - but happens during some sessions and browser restart fixes it for the new messages. It seems that these messages exist in the database, but are not assigned to their parent feed somehow and therefore don't show anywhere.
Yesterday I had a slew of feed items disappear (I had pop-up message and notifications in windows notification sidebar, but no unread counts and no messages in feed listed), browser was working fine otherwise with no visible issues, but restarting browser session lead to NEW feed items appearing in the feed messages lists.
So, here I my observations:
Hitting G while viewing specific feed messages list does nothing. Items are unread, but browser doesn't see them and doesn't skip to them while viewing their parent feed message list.
They don't show in unread counts anymore (at the time of my initial report I had notifications for hem)
Searching for part of feed news message title within parent feed won't find the message, but switching to All - will find it with proper author listed.
Flagging messages will show them in Mail Flags filters and here hitting G will skip through them as expected.
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