Why aren't the modified details of my contact reflected in my lead contact details?

You enter contact details such as phone and email on the lead record when you first associate the contact to your lead. If you later make changes directly to the contact record, those contact updates aren't reflected in your lead contact details.

This is expected functionality because the fields are different and changes made to the contact won't automatically synchronize to the lead contact record. You must manually update the contact details on the lead or get your application administrator to create a groovy script for the BeforeUpdate attribute to sync the updated contact data with the associated leads.