Once you rehab your loans the original tradeline (the one that defaulted) should be marked paid in full and any remarks relating the default should be removed. Some lenders will entirely remove this tradeline, or scrub it clean of the lates, but not all do. Any collection account related to the defaulted loan should also be removed from your credit once rehab is complete.
When the loan is transferred to a new servicer, they should put a new tradeline on your report for the new loan. It should be backdated to the original open date of the loan and reflect all of your payments toward the loan going forward.
Which tradeline has disappeared from your report? If it's the old, defaulted one with the lates that led to the default, you want it gone, even if it drops the AAoA, getting rid of those lates is a good thing. If it's the new one, you may want to contact your servicer and see why they aren't reporting your active loan to all three CRAs.
