I too am mystified by how poor tag search-ability is in Evernote, especially when compared to how it used to work.
I first used Evernote 10 years ago BECAUSE it had this feature (click to select multiple tags for searches) and stopped because it lacked another critical feature (the ability to export or print my notes efficiently). Since then, I keep coming back (in both paid and unpaid accounts) hoping that the export features have improved--and while there are some minimal improvements there, it's still not where I need it to be yet--and the tag search-ability has taken a huge step backward!
10 years ago, it used to be that one could easily select multiple tags at once for searches AND Evernote used to display the number of files within each tag in the navigation pane (without having to click on the "tags" button). That info was helpful because it allowed one to quickly see if a tag was worth including in a "multiple tag search combination " scenario. This is an essential feature. Now, there is no way to select multiple tags by clicking, and it's hugely time consuming (and awkward) to type in ANDs and ORs with the UI as it is now. It could be so much more. Tagging is easy. Searching should too!! This will bring people to Evernote from the competition!
Seriously though, I wonder why it isn't up-voted. I suspect this isn't up-voted solely because your users aren't using tags to their full potential. They don't even know what value tags can bring. Please consider prioritizing the improvement on the searching experience for complicated networks of tags. The image is the kind of tagging structure I use to find appropriate lesson activities for team teaching assignments. I currently have hundreds of assignments and activities tagged but am prevented from scaling up to thousands or tens of thousands of activities (and many other applications) until the tag-searching experience is improved.