Experienced on PC version, most likely the same on other versions too:
According to text, kingdom power level 21 requires to complete the tier 6 trial (previously all 7 challenges on tier 6). On a kingdom with no previous trials/challenges cleared (power level was already 10, when requirements were changed in 2018), it was marked as complete after beating trial 1.
I can not confirm, if this would also happen with other trial levels between 2 and 5, but would assume so.