If an Attack Modifier ability has a requirement that is not currently met by the player, then the ability will be greyed out on the command card and cannot be used manually (as expected). However, if the ability can be auto casted and if auto casting is enabled, then the attack modifier will take effect even if the ability requirement is not met, and even if the linked Attack Modifier behavior has a requirement that is not met.
Additionally, CAbilAttackModifier appears to entirely ignore its “Auto Cast Validators” field and will auto cast even if one of the validators is clearly violated.
When testing this, I made sure that the unit had only the Attack Modifier ability and not any Attack Modifier behaviors, so that’s not confounding it.