Legal, lawful…..?
Wrong words to apply really.
You’d probably struggle to find it to be unlawful - it is however inappropriate / bad practice / open to challenge and or criticism etc, etc
I think the premise of the question is correct, but it’s construct is a bit clumsy.
The PC should vote upon that which is properly recorded as the plans and details which have been submitted to the local planning authority not that which is handed out at the meeting.
What if the developer now doesn’t submit the amended version to the LPA - you will have recorded a vote of support or opposition to something which is not as submitted.