Never believe it 300% boost. It is just showing damage boost wrong.I somehow managed to get the Alchemist flask and with it i managed to get into Hardmode really easily, but after dying once, its damage dropped from 300% to 14% and now its barely able to kill normal slimes. How can i get its damage back up ? Do i need to kill all bosses again or reroll its stats with the goblin ? But i'w never even seen 300% damage increase when using the goblin to reforge items..
And from what I know, it can't get its damage lowered. After first usage of weapon, you will get maximum possible stats (for your progression).
Make sure you update tModloader, AlchemistNPC and Calamity mod to latest versions.