It's not a bug.

It's to keep it fair with Dual wield relics.

For example, Kikoku.

You can't wield 2 Kikoku, so Spharai being able to proc x2 isn't going to make sense when no other weapon can.


Now if other relics can proc on double/triple attack and Spharai can't, that's an issue, but being unable to proc on offhand more likely is how it was designed and not a bug.