Hopper minecarts weird behavior

1 minecart is filled with one stack of items. (all minecarts are on rails meaning they are perfectly aligned)
3 and 4 minecarts evenly pull by 32 items from minecart 1.
if minecart 2 is also filled, minecart 3 will NOT pull items from it until 1 is empty

minecart 4 should ONLY pull from 1 and minecart 3 should ONLY pull from 2
this is REALLY confusing and annoying and broke all the sorters in my world, now i have to spend
couple hours fixing all that