I had a similar issue with my SMS setup a few months ago and whilst Support couldn't help me with it directly, they asked me to put the same tags into an email alert instead and see what was being displayed. When I got the email, I found that a number of those variables have HTML tags like <b> </b> and that was breaking the payload's XML structure. I ended up using the rewrite function to strip them out. Not sure if your issue is similar, but as Support suggested to me, try the same variables in an email to see exactly that is being passed through. 🙂
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