missing 1 small metal in gaiden

[Caffeine]

did every achievement, except i'm still missing 1 small metal, i have no idea where as it seemed to be random troughout the game and there's no way to keep track of where i could have missed one ( if there's a limit per zone, but i can't find any info about it online), does anyone know how it works? i grininded the last stratum a ton with 999 luck and even with lower luck with no success.

[gatsu86]

It is a random drop, keep trying and eventually you will find the last one. You can grind any stratum for chests.

[Caffeine]

Finally found it.... never tought i would have to grind this one so much