It's not a bug.
This is the description of the skill: For 5 min., the targeted party member's P. Critical Rate, P./ M. Skill Critical Rate, P. Critical Damage, and P./ M. Skill Critical Damage +25%. Additionally, activates the Spirit of Iss effect. P./ M. Atk. and P./ M. Def. +20%.
So it gives the targeted party member's P. Critical Rate, P./ M. Skill Critical Rate, P. Critical Damage, and P./ M. Skill Critical Damage +25%, which is what you want for your yul, and it's a great boost.
The Spirit of Iss effect P./ M. Atk. and P./ M. Def. +20%, as the name says, it's a buff that appears on your iss, boosting the iss stats.