So after enabling/disabling each mod I have I was able to confirm that Traffic++ is completely skewing the Cab AI behavior. I haven't investigated other service vehicles, so I can't say anything about them. However, on the large majority of the cases I followed cabs, they'd leave the depot and make only one drive in their shift. After delivering this first passengers, they will simply despawn while planning their next job.

I am also using "Traffic Manager: President Edition" and "Rush Hour", and having both these mods enabled while Traffic++ is disabled will allow the cabs to function properly.

Thanks for the awesome mod, and I hope this problem can be fixed quickly!