Captivate 8 / HTML5 - text caption containing variables not displayed if used as mailto: link

Has anyone successfully used a text caption that contains a user variable to act as an email link?

Text caption contains a user variable $$vName$$ - that is selected and acts as an email link.

On publishing the text is not displayed, but move the mouse over the area and the cursor changes to a hand icon and can be clicked to send an email. The link is there (and is activated on clicking) but the text itself does not appear.

As an experiment I have also tried adding a simple spacer variable to the end of a text caption, e.g. Send E-Mail$$vSpacer$$ and that does the same.

However, if you change the link to a web link - the text is displayed, change it back to an email link and it disappears again. Has anyone else had this and fixed it?

Thanks,

David