Just grab a water bottle to get the advancement for 'brew potion'

NOTE: you can't get the advancement if you brew a splash or lingering potion. You may want to add that. so you get the achievement for any potion brewed.

This bug occurs because the game is checking for 'minecraft:potion'

instead of triggering by inventory change, something similar to enchanting should be done?

"trigger":"minecraft:enchanted_item"

likewise

"trigger":"minecraft:brewed_item"