Changing details for gmail account not posible

Hi guys,
I sold one IG account to someone and gmail doesn’t let them change phone number.
Everytime we try to change the phone number it always wants to send a code to the original phone number used to create the account.
I even removed any phone number,but still any changes i wan’t to make it asks to send a code to the old number.
Anyone encountered this?
I found this topic on gmail forums it seems to be a bug with Gmail.