I'm not surprised. It's one of the credit secrets that lenders and credit card companies fail to tell you....and yes, even FICO.
If you dispute an account, particularly an old account, it WILL drop your score. It can take months for you to recover.
I went to one of those vulture mortgage lenders early last year at Wells Fargo who told me first that he just wanted to get me into a home...and to make myself look better to the underwriters, to just "dispute everything on my credit report" in hopes that the inaccurate information would just drop off. It didn't. Instead, my score fell 28 points, putting me out of range for a mortgage that wasn't 10 percent interest with three points. At that point, he didn't want to work with me any more....So now I'm renting.
Personally, I wanted that WF lender hogtied and placed on a C-17 over Baghdad....and pushed out into enemy territory. Then perhaps he would have known somewhat about how I felt about not only his bad advice, but his abandoning me after it just made things worse.
Get a lawyer and dispute directly with the company....not the credit agencies, unless you just want to spend the next year getting back on track.