Thanks for the quick response, Mathias!
Short Answer: It isn’t. Its causing the posts to completely disappear on the category and tags pages.
It is not generating an error in the debug log.
It is not a conflict with other plugin(s)/theme(s). It does this regardless of any other plugin activations. As soon as your plugin is deactivated the expected, correct default behavior occurs again.
It only sends a 404 on the category/tag pages when the category/tag (as set in the permalinks) base is only one (1) letter i.e., ‘example.com/c/‘ instead of ‘example.com/category/‘. If the category/tag base is >1 letter i.e., ‘example.com/cc/‘ it seems to redirect to an actual post, if the Link Fixer plugin is active.
The browser “remembers” the 301 redirection when the Link Fixer plugin is deactivated.
The Link Fixer plugin also causes this issue, regardless of whether your plugin is active or not and also acts independently of other plugin activations. Similarly, when the Link Fixer plugin is deactivated the expected default behavior is again restored, regardless of other plugins that are active, including yours.
So these plugins have some action/hook in common.
Nothing in the .htaccess file seems to be affecting the issue.
Thanks!
-Brian
WordPress: (5.7-alpha-50001)
PHP: 7.4.13
PHP Memory Limit: 9 GB/WP Script 32 GB Total