There's also another well-known bug where you can take one of Wither's hirelings, respec as a warlock, cast the shadowblade, give them something to hold in inventory, dismiss them, hire them again.... and voila, they'll have a perma-shadowblade in their inventory, but it will always be the 2d8 one, even if you originally created it as an upcasted version.
It is likely that there's some root commonality to these behaviours, as indicated by the 2d8 dmg reversion in both scenarios.
