You will need to find which plugin caused the issue for us to be able to help you.
What you’re describing is other plugins you’re using interfering with the normal operation of Wordpress post custom fields.
We can help you figure it out, but the issue is with plugins that you’re using that affect the normal operation of Wordpress. Changing the WP Discourse plugin to accommodate such behaviour would not be wise.
Once you find the plugin that is causing the issue, I’d suggest, as a first step, to reach out to the creators of that plugin to tell them that it’s causing an issue. We can definitely assist with a resolution in whatever way we can, but we can’t fix bugs in other plugins.