There is a known bug posted on github with regard to Motherboard despawning immediately (or almost) after being summoned, if the Destroyer has already been defeated.
TL;DR Fight Motherboard prior to fighting the Destroyer
184.108.40.206 11:02, 2 March 2019 (UTC)The First Person To Apparently Read Bug Reports On Github
Motherboard didn't drop anything?
Fought the Motherboard, was a difficult fight since I'm a squishy summoner and pre-Plantera. Killed it, saw all the hearts and mana stars drop, but no items? is that even possible?