Removal of 13th month for employee with some managerial tasks

Good day. I have been working for my company for around 7 years now and I have been getting 13th month ever since. Around 3 years ago, me and a colleague of mine have been tasked to lead our company. My colleague became the general manager while I am more of a technical director. Actually since our company structure is almost flat, position is not that important.

Lately, my employer came across an article about 13th month and he's thinking of removing 13th month benefit from managers that includes me. My question is, can my employer just say that I'm not eligible to receive 13th month at this time of the year? Below are some of my points on why I should still be getting my 13th month:

1. I've been getting 13th month since my first year, even in my previous years in the same role/position as now.
2. Most of my time is still dedicated to programming, which is what our rank-and-file employees do. Though I have a say on hiring and firing of staff.
3. I'm included in the company's payroll same as the rest.
4. There's no signed contract that my position is managerial and that I won't be getting 13th month.

Pleas help.

If he actually do it, it can be considered as diminution of of benefits. Also, all employees are entitled to a 13th month pay

employee who have worked at least one month during the calendar year is entitled to 13th month pay.

Yes but it is pro-rated. Common computation is this:

(Basic Salary/12)x number of months served