Your prior letter was a DV? If so, it was not an FCRA dispute, and thus not basis to dismiss a subsequent FCRA dispute.
Regardless, they cannot dismiss a dispute related to failure to update the debt balance based on any earlier dispute or DV that was directed at the validity of the debt itself.
Apples and oranges.
A later dispute can only be dismissed as frivolous or irrelevant if "substantially the same" as a prior dispute they have already investigated.
The issue in your current dispute is their failure to have complied with FCRA 623(a)(2)(B), which requires any furnisher, once having reported information to a CRA, to "timely" update that information so as to maintain its current accuracy.
I would send them a direct dispute under FCRA 623(a)(8), clearly identifying the inaccuracy as failure to comply with section 623(a)(2)(B), which is different from any prior dispute of accuracy. If they dismiss as frivolous or irrelevant, they must do so within the 30 investigation period set under section 623(a)(8).
If they do so, then send an immediate formal complaint to the FTC for violation of section 623(a)(8) by dismissing your direct dispute without proper basis.