Coverage under the plan shall commence on the first day of the month coinciding with or next following the day that an employee becomes eligible pursuant to section 309.2(a) or (c) of this Part; provided, however, an employee on sick leave or maternity leave pursuant to Part 337 of this Title shall become covered effective on the date of return from such leave. In no case shall coverage be denied a person otherwise eligible by reason of any omission in reporting such an employee as a covered employee, and premiums otherwise payable in consideration of such coverage under the plan shall be paid to the insurer.