Arenanet has finally given us an OFFICIAL statement regarding Magic Find affecting the items gotten from chests and mystic forge:
Source: https://forum-en.guildwars2.com/forum/game/dungeons/Magic-Find-Chests/first#post597487
Source: https://forum-en.guildwars2.com/forum/game/dungeons/Magic-Find-Chests/first#post598159
You heard it here folks. Grab your tin foil hats and start spinning in your computer chair when adding random items you wish to combine in the Mystic Forge for a higher chance of getting a legendary precursor.
Isaiah Cartwright
Guild Wars 2 Lead Designer
They only work on creature kills, chests however have a better table then normal kills. We wanted to avoid the game play of killing a boss and then having to equip your whole Magic Find set before opening the chest.
Also magic find is calculated from when the creature is killed so it’s also not viable to use your normal gear set kill a boss then switch to magic find.
Source: https://forum-en.guildwars2.com/forum/game/dungeons/Magic-Find-Chests/first#post597487
Isaiah Cartwright
Guild Wars 2 Lead Designer
It does not work on the Mystic Forge, magic find only effects the loot generated from killing monsters. (this includes crafting materials, higher tiers of loot like rares & exotics… etc… )
Granted Zomoross is mighty and I’m sure he randomly gives more favor to those who are wearing a tin foil hat in real life and are spinning in their chair while clicking the forge button he is evil like that.
Source: https://forum-en.guildwars2.com/forum/game/dungeons/Magic-Find-Chests/first#post598159
You heard it here folks. Grab your tin foil hats and start spinning in your computer chair when adding random items you wish to combine in the Mystic Forge for a higher chance of getting a legendary precursor.