Custom plugin affecting Page display

I have created a simple plugin to create post types and taxonomies (full code can be found on pastebin here) to use with my custom theme. However, when I activate the plugin and refresh my permalinks (which are set to post name,) my pages stop functioning as expected. Instead, it appears as though they are just showing my default index.php template, which is strange, considering my file tree (note: I have linked to pastebins below, and have only included relevant files for brevity):

Theme
|– css
|– images
|– inc
|– js
|– languages
|– template-parts
| |– content-about.php
| |– content.php
| |– page-about.php
|– about.php
|– archive.php
|– index.php
|– page-about.php
|– page.php
|– single.php

When I deactivate the plugin, the pages display appropriately. I’d think with the fallbacks of ‘page.php’ and the ‘about.php’ template would be enough to kick in, but somehow it seems WP keeps throwing the default template at all requests when the plugin is activated (and after permalinks are refreshed.)

The site is hosted by bluehost at https://www.press10.me/lance

I have built a few themes exactly the same way (built on top of a simple post-type/taxonomy plugin and custom templates) with no problem, so I don’t know what I’m missing. I’ve been racking my brain for two days over this one! Any help would be immensely appreciated. The