A pumpkin can only be placed directly on top of another solid block, and nowhere else

Intended. "Pumpkins cannot be placed without a block under them. However, you can place them and remove the block directly beneath and it will remain in place." -Minecraft Wiki - Note that it isn't in the "Bugs" section of the page.

Duplicate of MC-1947 , please use the search function to see if your bug has already been submitted. Currently over 35% of tickets are being closed as duplicate.