You welcome, Josh. The link will not be properly formed if the custom tag *|SP:CONFIRMLINK|* is parsed inside an anchor tag. In fact, the default suscription template outputs it in plain text.
So far the only way to confirm a subscription is to copy/paste the given link on the email. As I really need to fix this I’ve searched for that string replace in your plugin code.
I think that the code, when into a link in html body is encrypted twice. By how you process this in func send_optin, it seems that the code is first generated and encrypted by the function to be sent later, and then again in html(), as it is considered a link (e.g. twitter or facebook links in the footer are also encrypted automatically). Make sense to track clicks.
However until a new release of Sendpress I will try to prevent certain links (for example, with a certain class atribute) to be reencoded in the html() foreach loop ??