It’s the *inseparable* part of Chalcedon that inevitably creates the perceived conflict with PSA and Mary. I say perceived because the entire debate is often based on mischaracterizations and gets so nuanced that it feels like linguistic differences from one culture to another; everyone gets hyper critical, and mischaracterizations become rampant. Which means we’re over an important target since the demons get so riled up.
The short of it: you either end up with Immaculate Conception, or you reject PSA, and if you try to avoid this dilemma (rejecting Immaculate Conception but holding PSA), you get de-facto Nestorianism.
This is an oversimplification, obviously, but there is something to it, and the history of reformed theology does show this (as many reformed groups ended up in Nestorianism, refusing to call Mary Theotokos).
Nestorianism was defeated long before the full throated PSA was articulated. Cyril held a substitutionary view but focused on theosis/deification, and victory over sin/death/devil, that our corrupted nature could now be *healed* because of Christ taking our place on the cross, overcoming death, ascending as a man into heaven to sit at the right of the Father.
