While searching for similar bugs I came across MC-2912. It is very similar, accept the blocks don't swap completely here.

In creative mode I put two different blocks (Stone and dirt) in my hotbar (slots one and two). Then I swapped them by hovering over the dirt (2nd slot) and pressing "1". The graphic updates as expected. I then try placing dirt (now in 1st slot), stone is placed. Placing stone works as expected. Also works with potions (and probably any other food-like stuff, if you could eat in creative mode), except you drink them instead of placing them.

Steps to reproduce:
1. Go in creative mode.
2. Put two different blocks in any hotbar slots.
3. Swap them by hovering over one and pressing 1-9.
4. Try placing the blocks. The one you were hovering over looks like it is being placed but then tunrs into the other block, the other block can be placed as expected.
5. Open inventory while holding block you had hovered over. Your hand will be animated as if you were swapping blocks. The problem is fixed by that.