Hello,
On your Site, the default Thank You page content is also not showing and there isn’t such an issue happening on our Test Site.
So, could you please once disable all the plugins that are not related to our Theme and then check back the issue again. If it gets resolved, then reactivate them one by one to see which Plugin is causing the conflict.
Then try checking with activating the Parent Theme on your Site too. If that resolves the issue that means that there is some Customization in your Child Theme causing the issue.
If the issue somehow persists, then please provide us the permission to completely debug your Site which includes deactivating all the third party Plugins and removal of on-Site Customization changes too, so that we can thoroughly check this issue on your Site and assist you accordingly.
Best Regards