QB Issue Resolution:

Do these merged customers show up by name in the customer center? You may have dragged some of them under another customer (which makes them a job – kind of a “sub customer). If that is the case you can drag them back out to the left and above the “parent” customer and they’ll be back on their own.

Resolution for Issue 'How do I unmerge clients?' available: Yes (Solved).
Source: Intuit Community forum.

How do I unmerge clients?: 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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