Mobs cross rail. Normally, mobs do not cross rail, but now, mobs do.
Steps of Reproduce
1.Put some rails
2.Summon some mobs nearby the rails
3.Wait a few minutes
4.They will cross the rails
Results
The mobs and a minecart collide because of this bug. In Java Edition, this phenomenon is prevented by preventing mobs from crossing rails.
I'm 95% sure this is intentional. Mobs do cross rails - do you mean spawn on rails?