The Zendesk article for Underspire mentions that Wandering Merchants can be encountered in the Underspire.
Note: You have chances for Wandering Merchant Encounters like you would in Delves or the Explore game modes.
I assume this is just a mistake on the article, and it’s intentional that we don’t encounter Wandering Merchants in the Underspire.
Or is it a bug that no one seems to have encountered one in Underspire? At least I haven’t, and no one I’ve talked to has.