QB Issue Resolution:

Select the customer in question, drag up and left. Watch the blue line so you’ll know when to release.

Resolution for Issue 'I have somehow merged two customers; one is now listed as a subset of the other, and is a job not a customer. How do I fix this mistake?' available: Yes (Solved).
Source: Intuit Community forum.

I have somehow merged two customers; one is now listed as a subset of the other, and is a job not a customer. How do I fix this mistake?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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