The weather app in iPhone has strange opinions about the daylight in my home town.
Verified that the same thing happens in at least three other iPhones when adding the city of Oulu in Finland to the Weather app.
Filed a bug report to Apple.
Update: the issue is clearly because here, at mid summer, the Sun sets after midnight and then rises a couple of hours later. Now (July 7th) when the Sun sets just before midnight, everything looks as it should. The weather app just can’t handle this.