Okay I'm able to reproduce on https://beta.flarum.site/
It appears the first time the discussion list loads, it shows the pre-loaded homepage content, disregarding any tag filter.
The same thing probably happens when clicking the link to "following" or "private messages" from the tags-as-homepage page. But those extensions aren't installed on the test site.
The discussion list data should probably not be pre-loaded when the homepage is set to tags. I wonder if it's an issue introduced with the last beta or not.