It is a bug and support is aware of it, but don't have it slotted to be fixed for another couple months. Here's a quote from my support ticket:
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After speaking to Product Management about this (I was trying to see how we can get this supported), they told me that in about two to three months, we will have the ability to use token in the Reply-to field within a trigger campaign. I wish it was sooner for us!
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However, we noticed that if you used the same token in the From & Reply To that it worked. So if you are okay with the emails coming from the salesperson, that could be a possible solution until this is fully supported by Marketo.