Line 12c, Code W. on W-2 Form

Long story short, I had an HSA account back in 2012-2015 and had funds left over in that account. I never used those funds, since I RARELY go to the doctor and now they are appearing on my 2020 W-2 and I am being taxed for them. My problem is, I CANNOT for the life of me, find who is reporting these funds to the IRS. I have contacted my former employer and also contacted ADP, who issued me the debit card back then and NOTHING. I have even tried contacting the IRS for help on figuring out who is reporting these funds, to withdrawal these funds and avoid from getting taxed again.