I'm not sure this is a bug. At least in the cases I've had, it seems to be by design.
In C:S 1, busses could easily end up driving in stacks, like if there was a traffic jam, and busses on a line ended driving very close to each other. A mod fixed that (the one where you could adjust bus types etc., don't remember the mod's name). The mod made sure that if this happened, the busses would take longer time to continue, to spread out more on the line.
In every case I've had busses in C:S 2 pausing for a long time at a bus stop, it was because of the same issue. A lot of busses on the same line driving too close together.
So to me, it seems, that the devs implemented the same feature, as the C:S 1 mod.
One of my love affairs in both Cities: Skylines games, was/is to solve traffic. I've spend many hours in both games, to watch traffic behaviors, and solve the traffic accordingly. So I feel confident this is by design, at least in my own C:S 2 city.
If you click a bus line and you see that in between two stops, you have 3 or more busses, the first will exit the next bus stop quickly, and one or more of the rest will pause for a while.