When ths SS law was change by Reagan. We were told that the people who started prior to a certain date would be "GRANDFATHERED IN". Meaning the new SS law would not affect us. (The city of Los Angeles Dept of Water and Power) workers if we should chose to work after retiring from the city. I retired in 1999, worked long enough to qualify for SS benefits. When applying I was told that my monthly allotment of approximant benefit $900 would be lowered to $200. Due to my working for the LADWP. (while working for the LADWP THE "grandfathered in" people DID NOT PAY INTO SS AT ALL) when I tried to explain the situation I was told "law is the law " and the person wouldn't follow up on it.
There are various 'grandfather' provisions in the Social Security Act as amended, but what you're apparently referring to is the Windfall Elimination Provision (WEP). The WEP provision can cause a person's Social Security retirement or disability benefit rate to be reduced if they also receive a pension based on their work and earnings that were exempt from Social Security taxes.
The WEP provision went into effect on January 1 1986, and in order to be exempt, or grandfathered, from the provision you must have been eligible for either your Social Security benefit or your non-covered pension prior to January 1 1986. 'Eligible for' is defined in the provision as meeting all requirements for the pension except for stopping work or filing an application (https://secure.ssa.gov/apps10/poms.nsf/lnx/0300605364#b). In other words, you would only be 'grandfathered' if you could have retired and started drawing either your non-covered pension or your Social Security benefit prior to January 1 1986.
I have no way of knowing whether or not you meet an exception to the WEP provision, but if you believe that you do you should probably consider filing an appeal of Social Security's determination to reduce your benefit rate (https://www.ssa.gov/pubs/EN-05-10041.pdf).