The issue is that, though your value resembles an object, it is still being treated as a string. I wrestled with this for a while but I cannot figure out a way using our Handlebars rendering engine to cast that value to an object, which would allow you to then look up the properties within it. I’m going to file a feature request for a new Handlebars helper that would allow for this use case (casting a stringified object to an actual object).
As a side note, would moving any of these device tags to attribute tags solve your use case? Those values are also available on the device context object just like device tags are.