Probably patients consent to using software to process their data and chatgpt is considered just another software in the stack.
Edit: parent poster said no info is sent.
that's not how it works, PHI and HIPAA usually supersede a lot of these. That's why you hospitals don't just have everyone opt-out data protections if they want services.
They probably consent to software that's been validated and managed by their medical provider. I can tell you for sure that I personally wouldn't consent to a a random script thrown together by an MD with leetcode experience that sends your medical data up to an experimental service.