I looked this bug up and I found very old reports from around 2012 which were resolved and could not find any recent cases.

When I die in this latest snapshot 18w48b I respawn at the world spawn point because I don't have a bed. I was fighting enemies underground. I went to grab my stuff and upon arriving I could not see my items anywhere (I wasted no time after dying). As well as my items being invisible the enemies were too (two skeletons and a baby zombie). I died many times because of this but I noticed every time it was random. Either one mob, two mobs, or all mobs would be invisible or visible upon each respawn. My dropped items invisibility was random as well but with the items it would be the ones I managed to collect before my previous death that were visible or all of them were invisible or visible. I'm not sure how else I can describe it besides it being a very strange bug and since this is a snapshot it's expected.