It was so painful to find out this. Deleting and reinstalling templates don’t reset the templates to the original one. I tried plugins like wp reset to reset theme, and tried database cleaner to find database table containing the customziation. I connected to the database in a command line and checked all the tables, but no luck.

So, here’s the up-to-date solution. Go to appearance -> themes -> customize -> templates. Then you’ll see a list of templates like:

Let’s say I want to revert single post. Open it, and then clear customization like this:

Sometimes, you may have your own patterns (see ‘Design’ screenshot above) for commonly repeated patterns like comments, header, etc. In such case, delete your own. That’ll break pages that are using yours and that was happening without you knowing about it. Delete your pattern and insert the original one, e.g., delete your own comment pattern and insert the default comment of your theme.