Minecart Dispensers Failing to Contain Carts

At each of my rail stations, I have a dispenser containing carts, above which is a button. Adjacent to the dispenser/button arrangement are a powered rail and button. Normal behavior is as follows: Push dispenser button, cart appears on track, ride to destination, place cart in dispenser. Since the 1.5 update, typical behavior is: Observe a clear track with empty station, dispense cart, collide into other carts that have suddenly appeared on the track, sometimes near departure point, but usually near destination. I assume this is a bug, unless the features of 1.5 prohibit my previous normal use. In summation, dispensers throughout my map no longer seem to contain minecarts when any single dispenser has been activated.