The bug

When one of the hotbar keys (1-9) is pressed at the same time as Drop Item (Q), the item being switched to disappears, while the item being switched away from is dropped. For instance, I had inventory slot 3 selected and pressed 1 and Q simultaneously. The item in slot 3 dropped on the ground, while the item in slot 1 disappeared.

I was able to replicate this issue in Creative, Survival, and Adventure modes. I was not able to replicate this issue by switching items using the scroll wheel instead of the 1-9 keys.

Could be related to MC-116618, as both relate to dropping items in quick succession, however that issue is described as occurring specifically in creative mode.