This is an intended feature, in creative when a player has a full inventory he will still pick up items and they will disappear, which works as intended. You can see that in survival the items don't disappear but fall on the ground. It is only logical that what happens in creative is the items drop on the ground but before they reach the ground they go into your full inventory and disappear, this also matches up visually
seems pretty intended, you can have floating blocks in Minecraft, it's not new, and the grindstone is not the only block which does this (see MC-204035), seems hardcoded into Minecraft.