I had to hire an accountant because of this. So first off I'm sorry you're dealing with it, especially if you're someone with an otherwise simple tax filing situation.
My understanding is this:
My accountant manually fixed the incorrect amounts (lowered my social security income to the max, and my social security taxes withheld to the correct max) and submitted my taxes as such using his pro intuit software or whatever he uses.
Separately we are mailing in the 843 with a copy of my conversation with Justworks citing that they refuse to correct the excess and citing them utilizing IRC 3511 allowing them to over-withhold social security.
It's a mess and pretty messed up to put the onus on the tax filer to fix this. It seems like the tax law needs to be adjusted to either prevent PEOs from over-withholding, or simplify getting the excess withheld resolved by more normal means.
Overall it seems like both parties are pointing to different rules to create an unclear path forward to resolve. In my case I landed on the 843.
