Some of the BTS composers and producers just contribute a bit to bts albums, but they aren't in house producers in hybe. JYP just listens to bighit groups albums and looks at the credits and then calls them to work on their songs, that's it. Cos they can work for anyone, they aren't exclusive to BTS at all. Twice used Fontana too.
HOWEVER, this looks like it's bad for itzy, jyp unfollowing them means they might not invest them in as much and probably has something to do with lia's thing and how they think all their "work" training them, investing in them went down the drain and now they will not put in as much effort. Hopefully they just unfollowed itzy because they don't want them on their timeline, I mean it's not itzy members posting on that account it's staff. So it's not personal.