Skeletons with given items turn invisible

I have a DJ skeleton to make discs for me. I gave him clothes and a bow so he didn't despawn.
After a bit he wasn't in his cage. I went to look for the armour and the bow he should have dropped. when I looked in the cage I was shot by an arrow. I had thorns 3 all over so the DJ died. I replaced him with another DJ thinking it was a one time thing. After a bit the second DJ also turned invisible.