In 1.21.8, I was able to reproduce it when following the steps.
Indeed, as Ners said, if you use dispensers to place multiple at the same time, they will sync up, but the time is still random. I sometimes got 10gt’s too, so maybe it was just a 10gt delay by chance in the case of Ners?
((also to clear up future confusion, it’s detector rails, not activator rails ^^))
also just to be clear, the item inputting was always synced up, it’s just the minecart placement that wasn’t before.