Items are not stacking on secondary hand in 15w31b

I farmed logs. I had 52 in my Inventory and I switched them to my left hand. I saw an Item and wanted to pick it up. but I got it into my normal Inventory and nor on the 52 in my left hand.

What happened:
I got items in the normal Inventory.

What I think should happen:
I get Items in secondary hand, if there is a uncompletely stack of the same Item.