QB Issue Resolution:

They should.  Do you have them in the customer contact section?  Are you using recurring transactions (and perhaps you need to edit>add email>resave) to refresh them.

Resolution for Issue '“How come we can’t have my customers email address auto fill when creating a new invoice each time? It is a nusance having to enter this in the everytime …”' available: Yes (Solved).
Source: Intuit Community forum.

“How come we can’t have my customers email address auto fill when creating a new invoice each time? It is a nusance having to enter this in the everytime …”: 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 add email
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Error Recovery for add email [Canada]
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