@rbennett We’re sorry to hear you are experiencing this issue on your site.
We attempted to replicate this error on a site and upgrading from 19.5.1 to 19.6 didn’t result in any pages/posts returning a 404.
Can you confirm that this was the update to 19.6 that caused this issue? Also, does the issue still occur with only a default theme such as TwentyTwenty and Yoast SEO active?
We’d recommend to test this on your development or staging site, if you have one. If not, we recommend using the?Health Check & Troubleshooting?plugin. This plugin allows you to run a conflict check?without affecting normal visitors to your site.
If you’re unfamiliar with checking for conflicts, we’d like to point you to a step-by-step guide that will walk you through the process:?How to check for plugin conflicts.