Similar (but not a duplicate) to my last question about filtering by multiple tags, I noticed that the "related tags" links no longer filter the original search by requiring the additional tag. Instead, they work the same as linking to the tag, so all questions under that tag are displayed and the previous search is discarded.
You can reproduce this by going to the bug questions (on Meta.SE) and clicking any of the "related tags" links. It will behave the same as clicking on the tag link (filtering only by that tag) instead of how it used to (filtering by both tags).
The original goal of the "related tags" was to allow for combining tags so the list of questions could be filtered down to what the user cared about. So I find it somewhat strange that you can no longer filter down questions quickly using the UI, even though the numbers next to the tags correctly reflect the combination of tags (even if an "any" filter is in place).
I should note that anonymous users have this same behaviour, most likely because they cannot filter down a question list by multiple tags. Maybe the system is grouping those with the new UI together with anonymous users, and that is causing the issue?