Stack Overflow Developer responsible for this bug here! I've been investigating a different way to retrieve the Posts-associated-with-a-Tag data, starting with the Newest-sorted data and, unfortunately, when I turned it on for a short duration on the Stack Overflow site it exhibited an incorrect behaviour which I hadn't seen in the testing that I had performed locally and on other sites.
The new code is behind a feature flag and so can be toggled on or off easily, which explains why the bug appeared and then disappeared and then briefly came back.
I'm confident that it is now working correctly (following a code deployment yesterday) and the flag is enabled again here (meta) and on the Stack Overflow site - so those sites are using the new code.
Thanks for reporting this bug and please accept my apologies for the inconvenience!