Problems in Published Posts and Drafts after upgrading to WordPress 5.6

This may be a plugin or theme conflict. Please attempt to disable all plugins, and switch to the default Twenty Twenty-One theme. If the problem goes away, enable them one by one to identify the source of the problem.

If you can install plugins, install Health Check. On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.

After enabling the Troubleshooting Mode in Health Check & Troubleshooting plugin, the problem still persists. The Health Check & Troubleshooting plugin says it has disabled all the plugins but they are still active and running because I see their settings and while editing posts their options are still there.

What could be the reason for this?

That’s odd. In that case then you’ll have to do it manually.

Please attempt to disable all plugins, and switch to the default Twenty Twenty-One theme. If the problem goes away, enable them one by one to identify the source of the problem.

The source of the problem was this plugin ImageMagick Engine

In that case, I recommend reporting the problem at https://www.ads-software.com/support/plugin/imagemagick-engine/ so the plugin’s developers and support community can help you with this.