Is it fair to reduce gratuity in employee contract amendments?

Dear Sir/Madam,

I have worked continuously for a company for 10 years and am now being retired. My original contract was signed in 2013. However, in 2018, we were asked to sign a new contract and were given a gratuity payment.

On 1 January 2023, we were given another new contract which further reduced the gratuity. Each new contract has reduced the gratuity from the original contract.

I understand that contract amendments should not disadvantage employees. Therefore, should the gratuity be paid at the original, most beneficial rate for the employee?