IMO, there were a bunch of things in play vs. the Bills in this regard. The Dolphins' run game doesn't scare anybody, and the Bills did not need to crowd the box, even on obvious running downs. They played a lot of cover two, with two high safeties (two of the best in the league, btw), so if a WR got 10 yards beyond the LOS, they were immediately into double coverage. The corners played pretty close to press man, which took away a lot of the short routes. Both starting LBs (Milano, and Edmunds) are exceptional in coverage, so even Tua's checkdowns were often covered.
And after Waddle, Parker, and Gisecki, do the Dolphins have a serious receiving threat? For the most part, the Bills had a four-man pass rush with one LB (Tremaine Edmunds) in the box. That leaves six defenders, often in disguised coverage, that Tua had a hard time accounting for.
I also noticed Tua staring down his receivers frequently. That gives defenders in coverage a huge advantage-- especially ball hawks like Tre White, Taron Johnson, and Jordan Poyer. It gives them a chance to cut the route. If Tua sees it happening, then there's nowhere for him to throw, even as accurate as he is. I'm honestly surprised the Bills didn't get more interceptions.