Could some advice me how it is possible that adding a bus to the line changed freq from 3 minutes to 9 minutes?

Lets have a train line “RedExpress” city to city 2 Red arrows have there 3 minute freq. Now make a bus stop in front of each train stations, group it. Next add a bus to the line “RedExpress” and … you have freq 9 minutes. 😮

The bus is in the “train line” but he is going by road. There is a road, of course. Anyway the bus should only rise the freq, not lower it. I simply dont know, how its possible.