It's intentional. He wants to pitch deep into games.
https://sports.vice.com/ca/article/how-discovering-a-sinker-changed-everything-for-marcus-stroman
"I'm a completely different pitcher than I was for the first 23 and a half years of my life," says Stroman, who must have started pitching at a very tender age. "I was a four-seam guy who had trouble getting to the sixth, seventh, eighth innings. It would be five, six innings, high strikeouts, homers, fly-ball guy. And now I'm the complete opposite."
Now he aims to pitch to contact, and the sinker, when it's working, often induces weak contact. Indeed, in the seven regular-season starts since he came back from knee surgery last September, he has averaged 12 ground-ball outs. In his first start this year, he had 15.