Minecarts with hopper with items in all slots but enough room to stack more of that kind of item, eventually fail to pick up matching items from the ground when other (not matching) items are present together with the matching ones.

Reproducing:

1. Create a powered rail track. Create a pile of items: about 20 stacks of stone and about 8 other "junk" items (in screenshot: cauldron, water bucket, comparator, repeater, redstone dust, observer, red terracotta) within a single block on the rail track - attached screenshot 2019-06-27_12.36.26
2. Summon a minecart containing 5 blocks of stone in its 5 slots onto the track ( /summon hopper_minecart 61 57 95 {Items:[ {Slot:0,id:"minecraft:stone",Count:1b}

   ,{Slot:1,id:"minecraft:stone",Count:1b},{Slot:2,id:"minecraft:stone",Count:1b},{Slot:3,id:"minecraft:stone",Count:1b},{Slot:4,id:"minecraft:stone",Count:1b}]} ) , screenshot 2019-06-27_12.34.59 and let it drive through the pile

1. Repeat step 2. about 5 times.

Since the minecarts have room for 5*63 items, all the stone should be sucked up. Instead, first 2-4 minecarts behave as expected (filling up with stone) but after that and with all consecutive minecarts we have the situation from screenshot 2019-06-27_12.37.42 - the minecart drove through the pile without changing contents, the stone is still present on the rail.  You can even stop a minecart right on top of the pile of items and it will still fail to suck anything in.

This is not 100% repeatable - may be a matter of performance. The more of the 'base item' is there the more likely it happens; with 12 stacks happened never, but with 40 always; 20 stacks is about 80% cases.