Merge Tag used in href on confirmation not working

When saving the confirmation with the piece of code below I get the following error.
“The text you have entered is not valid. For security reasons, some characters are not allowed.”

     <a href="{Hidden URL:4}" target="_blank" class="buttonAlt">View Resource</a>

This is being generated on a confirmation page. It tells me that it doesn’t like the Hidden URL to be on the href tag. Weird thing is that it works on one server but not another. One using 7.3 php and the other 7.4. Same versions of GF.
What is stopping that from working correctly?

Please see our documentation about the issue here:

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