Thanks for providing your report template. Could you please try testing a report output using the built-in sample template? Click the "Create sample template" button in the Manage Templates dialog.
Unfortunately I do not have a solution but I have notice that I am having a similar issue with producing reports from Survey123.
I am getting corrupted .docx files when they have more than four attachments and/or which and are over 334KB. So I am assuming it is a file size problem?
I am still trying to narrow down the problem and troubleshoot but it is rendering the macros I have to run useless.
Noticed there was some recent activity on this thread and wanted to put in my thoughts.
My workaround using Integromat/Make was to take the word document report and convert it to a PDF using the CloudCovert module. This has been working perfectly! Using the Survey123 PDF format always messes with the formatting.
The unreadable content error also occurs when generating the report directly from the Survey123 website. However I found that if instead of splitting multiple reports into separate files and use the Merge (Next Page) option, the resulting word document does not have this error.