Related Questions:
Why isn't my Intake Form showing results?
How do I get contact custom fields to update when filling in an inquiry form?
Answer:
If a guest is using the same email address, the new Booking will link to their existing profile, however, any Contact fields within their profile will not update. This is a security factor to stop any malicious users from potentially overriding existing Contacts information.
Depending on your situation, an alternative could be to use Booking Custom Fields instead of Contact Custom Fields.