You cannot spent the bonus action on an offhand attack if the opponent is already dead. There is literally nothing to attack. No P&P round would actually demand that you lose your bonus action in this situation.
That you have to toggle dualwielding on and off all the time in BG3 is one of the inconveniences of the interface.
Well... Technically, you can choose to attack the corpse. It will provide no benefit. But you
COULD do it (DM permitting that is)
As far as it goes in BG3...
It all comes down to how it functions and the reason it exists in the first place.
Since the only reason it exists, is to provide a "QoL" feature for dual wielders, enabling one button press to do a "Full attack" as opposed to having to manually select Attack and then Off Hand Attack sequentially.
That's it. That's the extent of its existence. It's not harkening back to any P&P rules. It's just, "It's slightly more convenient then having to manually input both actions manually"
The caveat is that due to the way it was implemented, by way of being a "Full Attack Mode" it doesn't actually function as the 2 separate actions that it is replacing. It only functions as a singular "Full Attack" that uses both attacks and thus both action points.
This can be both helpful (Such as initiating combat, where you get both attacks in immediately before combat actually starts - Which is especially useful for Assassins who get their action points back), and harmful (A target that dies after one attack still consumes the second attack)
Of course, it's possible to mitigate the downside, without toggling "Full attack mode", by way of manually using an off-hand attack for the situations where a target is only 1 hit away from death. As the "Full Attack" is only done when using the main hand attack command (Such a thing is also beneficial if a target is super low so that an off-hand attack without stat modifer can kill a target allowing you to use a stronger attack against a healthier enemy for any dual wield characters who haven't got TWF. In addition to any other situations where you might have an alternate use for your Standard Action - Such as Dashing or hiding)
