If you compare the list of periods of those two queries you see that period 660 is contained in both of them. This is not the case for US data. This seems to be a bug.
https://eu.api.blizzard.com/data/wow/mythic-keystone/season/0?namespace=dynamic-eu&locale=en_GB
https://eu.api.blizzard.com/data/wow/mythic-keystone/season/1?namespace=dynamic-eu&locale=en_GB