My ex company underpaid my cpf for 2020 as i left in may and i only realised the shortfall last month when reviewing my cpf. As i have some months with commission.?

From CPF:

Employers have the legal responsibility to pay the CPF contributions correctly for their employees. Any employer found with CPF non-compliance will be guilty of an offence under the CPF Act and would have to pay both the employer’s and employee’s share* of CPF contribution arrears plus penalties.

*Employers cannot recover the employees’ share of CPF contributions which are more than 6 months from the time the contributions should have been recovered.

As such, has it been more than 6 months?

Most importantly, I think it's better you follow up with the CPF Board and the relevant authorities on the next course of action and not follow up blindly with your ex-company. I mean if they can underpay your CPF in the first place, one can't help but wonder if they even know what they are doing now or whether they are trying their luck.

Cheers.

- Jeffrey (jeffreyleezq.com)​​​

My suggestion is to either clarify and confirm the situation with the CPF officer (rather than to settle on a "up to you" answer) or to seek legal advice. Given that legal entanglement, I doubt many people can give you proper advice on what to do.