Multiple Tag Query not working

I am having issues querying posts that contain multiple tags. For example all posts that contain the tag biennial AND hairy stem. It’s just for one of my WordPress installations though (I’ve been working pretty hard on it and don’t want to reinstall). The others work fine.

Here’s the site: https://essmextension.tamu.edu/plants-test/
Multiple tag link: https://essmextension.tamu.edu/plants-test/tag/biennial+hairy-stem

If I try /?tag=biennial+hairystem it still doesn’t work.

Any thoughts?