UPDATE: I got it. There is no bug here. Somehow got confused. 40 minute interval includes more of higher intensity work than 30 minute one.
Take a look at this workout personal records graph:
It shows 206W for 30 minutes and 209W for 40 minutes.
How is that possible? In my understanding it should be at least 209W for 30 minute interval.
I see this problem also in other workouts.
I already wrote to email@example.com but decided to take it also here.