I have found a solution, though may be particular to me. I’ll share in case it’s not.
I was using the method designed for “external url”s (see documentation) – it did seem to be working but perhaps there were changes in a recent upgrade that stopped it working. I have now changed to use the internal url parameter method which works no problem. That is:
>Use the “modal-link” URL parameter
>Example: https://wp-post-modal.allureprojects.com/?modal-link=https://wp-post-modal.allureprojects.com/modal-content