For Google Workspace.

For Microsoft 365.

Question:

Is there any naming rule for the generated Password Protected Zip file when attaching multiple files on one mail?

Answer:

The name of the encryption objective file which first appears after the mail source analysis is to be used as the ZIP file name.

Please note that:
The structure of the mail generated after the mail source analysis (file appearance order) depends on your using mail client environment.

Consequently, this appearance order should not be explicitly specified.