In case anyone comes to this years later as I did, I was experiencing the same issue in 2025. The problem was that though I’d created a module for emails, I had not toggled Use programmable email module on:
This probably wasn’t available at the time of the initial post, but this solved the issue of trying to split a contact property string into an array by a delimiter, but having the entire string end up as the single value in the array, instead of being split.