QB Issue Resolution:

You need to confirm your Windows Language, Region, Date and Time formats set to US English. The OS settings, when you buy QB for US, should be set to US.

Resolution for Issue 'How do I change my date format in Invoicing back to printing as MM/DD/YYYY?' available: Yes (Solved).
Source: Intuit Community forum.

How do I change my date format in Invoicing back to printing as MM/DD/YYYY?: 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.

Support for Date and Time formats
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Error Recovery for Date and Time formats [Canada]
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