Randomly many users reported that email with attachments opens on first attempt but on subsequent attempts it fails

Question

Since last 2-3 weeks many users of our organization have reported that some of the emails for both internal and external senders they receive with attachments, they are able to open on first attempt of opening attachments but on subsequent attempts, attachments size reduced, and they are unable to open it. this is happening with excel, word, pdf, ppt attachments.

We use third party email gateway (Proofpoint) and they have already reviewed and confirmed that issue is not from their side.

Ongoing case with Microsoft since last 2 weeks, but no avail yet.

MS suggested, to modify exclaimer rule as per following article, but we do not have one configured as we are bypassing Exchange online protection for all emails.

https://learn.microsoft.com/en-us/exchange/troubleshoot/email-delivery/messages-with-attachments-are-not-delivered

Hoping to get some answers on this.

Thanks