Using Form Processing Steps to Update Custom Data Objects

I'm looking for a solution to the following use case:

I have a CDO ( CDO A) that is preloaded with a set of Unique Serial Numbers.

I also have a Landing Page and Webform that is set to capture a Serial Number passed via a Query String.

There are 2 form processing steps that are set up:

1. If the Serial Number passed through the Form matches a Serial number in CDO A, then CDO A is updated with the form submission data.

2. If the Serial Number passed through the form does not match ( not found) in CDO A, the processing step is set to Update CDO B.

The objective is to have 2 mutually exclusive CDOs for matches ( customers) vs non-matches ( New or Leads from a different source).

I'm noticing that while CDO A is updated correctly, CDO B gets updated for matches and non matches.

Has anyone seen a similar issue? And, is there a workaround?