“If you turn my plugin off, do your regular blog posts show up on your tag pages?”
Yes.
“Is any code your running in themes/plugins using the pre_get_posts filter?”
Unfortunately, I am not a developer and I don’t know how to look for this. I asked my theme developer, I will let you know when he responds.
I do not see how this can be an issue with any plugin as when I turn all of them off (except yours) this issue still exists.
Also, I investigated further on my own, and it seems to me that the issue is that if there are no posts tagged (only pages) with any specific tag – tag pages show a blank page. For example:
1. Go to https://www.humanpopulationacademy.org/tag/images/
2. In the list find ‘People Born On July 14th Of Common Years’ (which is a page, not a post)
3. Click on ‘People Born On July 14th Of Common Years’ and at the bottom of the page note tags ‘images’ and ‘born in july’
4. Click on tag ‘born in july’ and note that nothing shows up
Also:
a. if I turn your plugin off, https://www.humanpopulationacademy.org/tag/born-in-july shows “Nothing found” and a search option
b. if the plugin is on, https://www.humanpopulationacademy.org/tag/born-in-july shows a blank page