According to this web site https://www.verywellfit.com/is-tea-gluten-free-562355 Starbucks says that some of their Tazo flavors contain gluten in the form of barley malt. It’s listed as “natural flavors”. So what’s the deal in general with “natural flavors”?
Natural flavors are not a problem. Do a search on the top right here for natural flavors and you’ll see some threads about it.