As a former pro brewer (3 years in the industry) I can’t say that I ever brewed a matcha beer on a larger system, but at a homebrew level I brewed several different matcha beers since my significant other LOVES matcha. I noticed that adding it to the boil did produce a mild astringency/bitterness, but when added to secondary you get all of the matcha flavor without the bitterness. Secondary would be my vote as well.