Extra attack doesn't specifically identify as a "standard"

It's a passive effect "attack". It'll randomly "attack" a target and cause tag.

Additionally, the passive itself is a different form of action as it's not his standard, while other characters just do "additional standard"

I can see it as confusing, but it's working as intended.

Unfortunately, there's little to nothing we can do.

However, based on the results we've received and can visually confirm.

There's plenty of players whom actually received Minato through this event, Unfortunately, Luck does play a role into it.

Hopefully, you get a great reward next time.