If the loan was not part of the repayment plan, then it wasn't discharged, because in a Chapter 13, any debt not provided for in the bankruptcy plan survives the Chapter 13 discharge order.
So, the first loan is still collectible, assuming that the lender sells the debt to a debt collector -- which could happen in the lender's bankruptcy.
If your name was on the first mortgage, then the lender can report it. And, since it wasn't part of the bankruptcy, there was never any injunction to prevent the lender from continuing to report it negatively on your credit report.
The only way that you could get it removed completely, other than to wait out the seven year statute of limitations
on FCRA reporting, would be to contact the original lender and see if they have sold the debt. If they haven't, then you could offer to pay $X in exchange for a "release" from further liability, which would include a "recital" in the release agreement that you were always paid in full and timely paid on the loan.
I have done this before with an auto loan, but not with a mortgage. However, there is nothing special about a mortgage that would prevent the lender from doing this transaction. If the lender were to agree, and you received the release with the recital, then you could submit that to the credit reporting agency, and they would be required to delete the negative entries, because the lender will have admitted that you were always paid in full and timely paid.
That's the only thing I can come up with that may work. But, if the debt is already sold to a third party debt collector, then that would terminate the lender's negotiating powers, and you will be stuck with the negative entry until the seven years expires. 15 U.S.C. 1681c(a)(2).
Hope this helps.