QB Issue Resolution:

If they are pending now, they aren’t affecting your financial statements or the quantity on hand.

If they had been final builds, then changed to pending as you describe, THAT is when things were affected. When you issued the build (not pending) the parts were consumed. When the build was changed to pending then the parts were returned. It is as if the build never occurred.

So it is safe to delete them.

Resolution for Issue 'Can I safely delete old pending builds if they were finalized builds at one time?' available: Yes (Solved).
Source: Intuit Community forum.

Can I safely delete old pending builds if they were finalized builds at one time?: 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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