Items can render invisible under certain fire conditions.

To reproduce:

1. Stand on a block and run the command {{/summon item ~ ~ ~ {Item: {id:"minecraft:dirt",Count:1b}

   ,Fire:0s,Invulnerable:1b,PickupDelay:20}}}

2. Back away immediately to prevent picking up the item.
3. Light a fire at the item's position. The item can still be seen within the fire.
4. Put the fire out.

The item will become completely invisible. It can still be picked up by getting close to it.

Through testing, this seems to be related to that Fire tag, but I'm not 100% sure.