Thank you for the test you have done and the information given.
I double checked your website and added a sample Modal in the ConvertPlus. Then added the second test page and added both the test modal and your modal. You can see the link of the test page in the secure note.
The test modal shows with no problem and your created modal does not. That shows there is a problem with the modal that you have created and you need to recreate that modal from scratch.
Thank you for your understanding.