I don't care for a fix, just care if you fully understood and note the problem for a future fix. Because it is easy to get lost in all the topics you monitor. I do know and understand that, doing some similar job as yours myself (hosting support).
If you want, i can easy replicate it HERE so YOU get also the false notification.
We fully get the issue. As far as we fill HTML in the email notification content, it was substring the content (the <a> tag was opened and closed on the wrong place) and somehow the support topic linked and unsubscribed liked were missed.
If the email content is not filled as HTML or ignores the substring function this kind of issue never takes place. So the content will be unlimited.
In the second case the following code should be used. So the limit will be ignored: