Panda, you know just because the chance drop is for example 60% doesn't mean that you will get always 6 heads per 10 wither skeletons.
I personally haven't noticed an significant decrease of it to be honest but increasing the drop rate, taking in count how easily the mobs in Nether, specially wither skeleton, despawn due to the most random situations, would help in said grinding.

farming wither skeletons is really easy with a wither skeleton farm and the current droprates are op so id almost say that the skull drop rates should be decreased

sasuke dont mansplain probability to me, that was clearly a snapshot of the most recent data. its been a problem for a couple weeks now

i also dont understand your last point about how despawning wither skeletons would help with grinding, those two things cant both be true at once

ive been farming wither skulls since September and my drop rates are fine.
try double checking your spawning areas and your sword that your not missing anything

using the same sword as before (beheading rune but I don't remember the rest) and I'm in the same area