There are many reasons of why this can be happening. Most of times it’s because your theme is not use “the_content” filter which is needed in order to protect this information. It happens with themes that don’t follow the WordPress Development Guidelines.
The easier way to check if your theme is following those guidelines is switching to one of the default WordPress themes and check if the content is being protected. If the content is being protected properly then you will have an answer to your issue. Somehow your theme is not using the_content or it’s being overriding by one of your plugins.
We recommend you checking the Troubleshooting Guide.