What if v config is used on an x frame?

If there’s a payload weight in the front, should v be used?

No, in terms of stress on the front motors there’s no difference between X and V.
And if you think about using the V configuration to use bigger motors and props in the front, then still no.

you cannot use different size motors, it doesnt work. basically, you would need separate pid loops for each size of motors.

The flights have been flown, and I’m reviewing the logs. The frame is an x frame. But the config is set for v. Would that cause problems in pitch and roll.

Maybe. Maybe not. Even if it doesn’t cause problems it is not good practice.
What is preventing you from setting it to X?