I count ~600k items visible in the buffer, but I'm assuming it can only show so much. I have an EnderChests Ender Chest on top, my system on the other end was full, but it just kept mining.

I tried looking for documentation prior to creating this issue, but can't seem to find any.

It's intended in all version, platform.

Quarry can hold all items it mined. The items are saved in world, so kept in loading/unloading the world. However, you can't get items if you remove the Quarry block. Then items will go to the void.