Here is the law
PRESIDENTIAL DECREE NO. 851
REQUIRING ALL EMPLOYERS TO PAY THEIR EMPLOYEES A 13TH-MONTH PAY
WHEREAS, it is necessary to further protect the level of real wages from the ravage of worldwide inflation;
WHEREAS, there has been no increase in the legal minimum wage rates since 1970;
WHEREAS, the Christmas season is an opportune time for society to show its concern for the plight of the working masses so they may properly celebrate Christmas and New Year.
NOW, THEREFORE, I, FERDINAND E. MARCOS, by virtue of the powers vested in me by the Constitution, do hereby decree as follows:
Section 1. All employers are hereby required to pay all their employees receiving a basic salary of not more than P1,000 a month, regardless of the nature of their employment, a 13th-month pay not later than December 24 of every year.
Section 2. Employers already paying their employees a 13th-month pay or its equivalent are not covered by this Decree.
Section 3. This Decree shall take effect immediately.
Done in the City of Manila, this 16th day of December 1975.
Nowhere in the law that your 13th month pay should be withheld for any cause, otherwise it shall have been stated in the law.
Withholding of your 13th month pay is therefore illegal because it is contrary to law. Please report this to your HR. Supply her a copy of the 13th month law.
Hope this helps!