Stuff lost after death by Wither

I was trying to beat the Wither boss, unfortunately it beated me ! 😞
I went back to the place I died in 2 minutes but I couldn't find my inventory. The XP was here but no trace of diams armor and legendary sword ... :'(

I tried to suicide with a creeper and this time the stuff was dropped. I think the problem is specific to the Wither Boss.

I don't know if I am the only one with this problem but I hope you will be able to help me. 🙂