The behaviour that you describe, concerning attachments not being visible within the native iOS/iPadOS Mail App, appears to affect some types of mail message. While the majority of mail messages and attachments are not affected, for mail messages that are impacted by this problem, there doesn’t appear to be specific fix.
Something within the specific email formatting, or the attachment type, doesn’t play nicely with the iOS/iPadOS Mail App.
While a fix remains stubbornly beyond reach, there are workarounds:
1) Use a different Mail Client App to access your email accounts
2) Access the offending email message(s) via your Mail Provider’s webmail portal via a web browser.
Notwithstanding the above, you would be well advised to submit a Bug Report to Apple via its Product Feedback portal. For iPad and iPadOS:
Feedback - iPad - Apple
Every report counts - as until Apple receive a sufficient number or reports, it is perhaps unlikely that resources will be assigned to investigate and resolve the issue.
When submitting a report, ensure that you include as much detail as possible within the body of the report - including your iPad model and installed version of iPadOS; don’t rely upon the pick-list, as this rarely includes the full range of device models or most recent versions of iOS/iPadOS.