We may have found a bug with Outlook 365 and wonder if anyone else has seen this behaviour.
We receive .XLSX report attachments from an external domain by email on a regular basis and have recently needed to use the Outlook advanced search feature in Outlook 365 for 'Attachment Contains' to find some text in these attachments. We were surprised to find that these emails could not be found.
After review with our IT department they ruled out the following:
Repairing Office and recreating Search Indexes on my Windows machine did not fix it.
No related group policy for Outlook Indexing on machines that would affect this.
Indexing of .xlsx file is set on machine and that the issue persists on different machines
Indexing of .xlsx file is enabled on Exchange settings and is working for emails sent by internal users/ usermailboxes. The issue only persist if sent by an external domain.
We found if the external domain send the same report in .csv file format then the search function does work correctly. This has become our workaround.
Our machine is a Windows 10 Enterprise machine v1909. Outlook is Outlook for Microsoft 365 MSO (16.0.13801.21072) 64-bit.