CA child support agency terminated my case before child turned 18?

I recently received a letter from DCSS, county of orange stating the following:

"Dear B***** B******:

This letter is to let you know that on 05/10/2020 the local child support agency terminated child support services and closed the support case for the children listed below:

L***** T*****....etc,etc..."

What reasons would they have for closing the case before the child turns 18?

-He is only 13.
- I tried logging into Customer Connect, and got the response: "The info entered does not match our records"
- When i mentioned this to the mother, her response was: "They mentioned it had something to do with having an invalid mailing address on file"
Could this be true? If so? would it wipe out what i owe, making my balance $0 or $301?? (See below)
- When i contacted DCSS by phone, they said i had two mailing addresses on file - my current address, and my previous address. Ive received plenty of statements to my current address. They also said that they showed me owing 2 different amounts of money: $0.00, and $301.00. Either way, i previously owed upwards of $49,000! They also said that information on the case closure was confidential and would not tell me why the case was closed.
-No DNA has been taken
- i defaulted on the paternity hearing
-i knew nothing of the kid until he was 2 weeks old
-the mother most recently asked me to sign paperwork to have the childs last named changed (to her husband's whom she has been married to since 1 month after the child was born)

Any advice or insight would be greatly appreciated.

San Diego Guy, 37