Hi Tom,
Are you changing them using the Typography options in the Customizer?
Yes.
When you update and go into the Customizer, do the actual settings revert back to 17px, or is it only on the website?
They have changed back to 17px in the Customizer as well as on the website after the GP Premium update v. 1.2.77 / Theme 1.3.21 (for: Typografie > Widget Typography, Typografie > Footer Typography).
Thank you for looking after this 🙂