Well, I think the problem was that he was allowing home runs with those tertiary pitches that he should have been throwing less of. It's true that 20.8% of FB's being homers in Chicago compared to 4.5% in LA & 6.8% in Boston is definitely a large departure. Part of that may have to do with the pitch mix as well in terms of the occasional hanger. But it definitely wasn't just home runs. Opponents had a .295 BA (.393 BABIP) against him with the Cubs, .133 (.200 BABIP) with the Dodgers, & .236 (.301 BABIP) this year with Boston.
Martin has not allowed a homer on a cutter since 2019. That's 696 total cutters in the past 4 years.
During that 4 year span, he has allowed:
1 homer on 275 sinkers (1 every 275 pitches)
1 homer on 267 split finger fastballs (1 every 267 pitches)
8 homers on 1,002 4-seam fastballs (1 every 125.3 pitches)
2 homers on 178 sweepers (1 every 89 pitches)
1 homer on 84 curveballs (1 every 84 pitches)
https://baseballsavant.mlb.com/savant-player/chris-martin-455119?stats=statcast-r-pitching-mlb
So it kind of makes sense if he reduces the sweepers & curveballs, he'll reduce the home runs, even though those are obviously small sample-sizes. But he threw 0 curveballs this season & 29 sweepers. The sweepers & the 4-seamer were his least effective pitches. He did throw 10 sliders, which were his 1st ones since '19, and those were very effective, but that is the tiniest of sample sizes. Both of the homers he allowed this year were on the 4-seamer.
Lots of ways to play with the data, & the bottom line is who really knows? & regardless, even if we did know the Cubs didn't maximize Martin, how much of that would be on Breslow anyway?