Nether only, seems to work okay in the Overworld
When making a Wither Boss in the Nether. it leaves 2 Soul sand, and 1-2 Wither Heads.
In the Overworld it seems to work fine and as intended.
tested in Peaceful, Easy, Normal, and Hard. in Creative and Survival and even Hardcore.
Glitch might also effect The End.
DISCLAIMER: Before you mark as a duplicate;
I searched for the wither and found that all bugs like this were in the overworld and have seemed to be fixed. This is a report saying that this affects the nether and the NOT the overworld