After a support case with Microsoft, they confirmed that it is not supported to do prechecks on ROPC flows. This is a blocker for many scenarios with B2C and existing ROPC flows (expecially when migrating from a different IDP). I ended up creating a pod that intecepts the request, do the prechecks then forwards the request to B2C. I used it to migrate users from a different B2C before authenticating them on the target B2C when using ROPC.