Title, unfortunately I was not recording when this occurred so all I have to go off is a run history image.
The bug occurs when the player, while holding Larva, takes a death with a stack of any Void Counterpart normal item topped off by any amount of temporary items. I don't know exactly why this happens since a temporary item picked up when a player has the relevant void item does become a temporary void item, but I thought it may be a bug.
In this run, I died to Solus Wing while I had a temporary Lens Makers' Glasses from a junk drone. As you can see, every other Void Counterpart item I had at the time did change.
I really, really want to think of this as a unique tech that a player could deliberately take advantage of if they had a reason to want to tank a death with larva (to Eclipse fall damage for example), AND had a temporary item on-stage they could use to protect an item they didn't want to shift. Still, it could be pretty confusing to a player who died with Larva and now is wondering why their Ukes didn't turn into Polyltues or etc.