Why does a payee drop from the favourites list especially after making a payment to the payee?

If you attempt to create the same payee (same sort code and account number) that you have in your account, then the favourite payee status of that payee may not be maintained.

If this happens then reset the payee as a favourite payee by tapping on the payee to open the payee details. Scroll down to the bottom of the page until you see "Add to favourites", tap on the star to highlight it.