It's similar to MC-168511 and MC-153901
In survival mode, i killed more than 100 wither-skeletons myself by a diamond sword with looting level III. They dropped lots of coal, bones and stone swords, bur no wither-skeleton-skull at all. When i at last time tried this in 1.13.2 i got about 5-6 skulls. Not tried in 1.14.x
So please check, what is going wrong.