Not a bug but a balancing issue.
I’ve recently added a kind of bus terminal to my airport with 11 stops in addition to the subways that I already had I currently have at about 40 people queuing at each stop but some stops have over 100. In the meantime the number of people using the subway has fallen from about 190-200 per hour to nearer 50 per hour for each entrance even though the subways are closer to the terminals. There are also very few buses coming to the airport relative to the number of people queuing.
As far as I can tell people prefer buses over subways far too much and it needs re-balancing in favour subways more than it currently does.