Player drops set to invincible when dying in the nether can burn and get destroyed. If it is a specific item set to not burn, the item will not burn and will work fine. This works as expected in the overworld.

Forge or Fabric?

fabric for both reports

this seems to also be an issue when dying in the end

Reproduced. How strange.

True cause: Invincibility/etc state of items is not saved. If the chunk unloads, the special item despawn rules will end up being ignored.

If you had a Respawn Anchor right next to where your items dropped, then this wouldn't occur.