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When a foreigner leaves a job in Singapore, the employer must withhold the last month(s) salary, pay all tax dues after IRAS calculated it, and then return the remainder to the ex-employee.
So all this worked correctly in your case, except the last part. You must give the employer sufficient time for the reimbursement (I suggest a month after IRAS clarification) and then ask them (in writing) to return your money. If they don't you can sue them for it - in this case it is good to engage a lawyer.
Good luck!

To add what Beppi said. IRAS only issue refund money via cheque to the name of the employee. It never refund to the employer’s name. I never come across a case where iras refunded to the employer’s account.

If the employee is still in Singapore (after switched to another job), then only iras issues a cheque or else they will credit to your mentioned local bank account.

So, check your residency letter box if didn’t receive them check with IRAS if they had send to your correct address or not. If they didn’t refund then they will adjust in present financial year tax assessment.