I'm curious about we think is most damaging on this front. Is it hitters getting a literal view of the fastball? In that case it feels like a very fine margin, because if hitters see 15 pitches across 3 PA against Steele, a 15% increase is 2 pitches. And if it's about the team's whole exposure to the pitch(and they're getting some view from the dugout or second hand experiences), that feels like an even more diffuse phenomenon.
The other thing I wonder about(and I'm not sure how to quickly check) is how much this is a reflection of command, and the ebbs and flows of fastball use(especially to RHH) may be reflective of how many 1-0s Steele is facing v. 0-1s, etc.