You have to use "slot.container.0" with the new /replaceitem command to access the Players Hotbar

I expected when i used "/replaceitem entity @p slot.inventory.0 minecraft:stone 1", that my first item in my Hotbar will be replaced with stone. But it replaced the First Item in my Inventory. I'm not sure if this is intended or a bug.

Greeting,
Balckwolf