Updating furnace snapshot in FurnaceSmeltEvent dupes items

What behaviour is expected:

I can modify the furnace snapshot and then update the block just fine.

What behaviour is observed:

Just getting block state snapshot and updating the block causes it

Steps/models to reproduce:

Use following code (Compiled plugin: toiletpaper.jar)

class FurnaceSpeedListener(private val plugin: ToiletPaper) : Listener {
    @EventHandler
    public fun onFurnaceSmelt(event: FurnaceSmeltEvent) {
        plugin.logger.info("Let's smelt!")
        val furnace = event.block.getState(true) as? Furnace ?: return
        if (furnace.update())
            plugin.logger.info("Furnace updated")
        else
            plugin.logger.info("Unable to update furnace")
    }
}

If you'll get the state without using snapshot, the code works just fine.

Plugin list:

Just the plugin listed above

Paper version:

[11:54:26 INFO]: This server is running Paper version git-Paper-314 (MC: 1.16.4) (Implementing API version 1.16.4-R0.1-SNAPSHOT)
[11:54:26 INFO]: Checking version, please wait...
[11:54:26 INFO]: Previous version: git-Paper-312 (MC: 1.16.4)
[11:54:26 INFO]: You are running the latest version

Anything else:

If you take out the smelted item, new one will appear - but the input slot never goes down, and never more than 1 item appears in the output.

Start by reproducing the issue with the FurnaceSmeltEvent listener and the event.block.getState(true) followed by Furnace.update(). Inspect the FurnaceSmeltEvent handling and the snapshot/update path to compare it with retrieving the state without a snapshot. Done means updating the snapshot no longer duplicates the smelted item, the input slot decreases, and the output remains correct.