This has always confused me too.
Investopedia say "because the forex operates 24 hours a day, there is technically no stoppage of trading between one trading day and the next as there is in other markets. As a result, the convention is to consider a forex day to be from 5pm EST to the same time on the following day, and most daily charts are displayed this way."
Daily Chart Definition | Investopedia
So if I have understood that definition correctly, the 2 hours of Sunday evening trading should be treated as the start of a new 24 hour trading period, rather than combined with Friday data.