boosting the base attack value should have no bearing on percentage based damage. It only changes when effects like Ferocious Strike, Ferocious Defence, etc are applied. and even then, it is still an increase to the percentage done. so grypo’s 25% rending counter does 37.5% with the ferocious buff but increasing its base attack with boosts does not affect the amount of rend damage done.
Marsupial Lion doesn’t have damage increasing moves to apply so its rend damage will not increase. ( its crit chance will but not attack) you may have been seeing its counter attack go up with boosts. if not, then that is a bug and will probably be fixed.