Dear friend,
If any employee whose monthly salary does not exceed Rs. 21000/= works for not less than 30 working days during an accounting year in an establishment to which the Payment of Bonus Act, 1965 is applicable, he is eligible to claim statutory bonus as per the provisions of sec. 8 of the Act on par with other employees who are in service on the date of disbursement of bonus pertaining to that year.
Therefore, it is not necessary that he should be in the employment of the establishment on the date of payment of bonus. Certainly, you are entitled to bonus for the AY2018-'19 since you had worked upto March2019.
In case of refusal on the ground of your resignation, you can file a claim u/s 21 of the PBAct,1965 before the designated authority or raise a dispute u/s 22 of the Industrial Disputes Act,1947 before the area conciliation officer.