This issue is most likely related to how the account was setup initially.
If you want to see all attachments, please set the `raw_file_list` parameter to 1 when you add the email account. See related documentation here.
A filter for the raw file list was applied when the 2.0 API was first introduced. We now realize that excluding this list by default was the wrong call. We've considered making the filter disabled by default, but unfortunately too many existing clients would need to update code on their end. Therefore, we've kept it as is and will switch the default in the next version of the API.
Please note that the Lite API always shows all attachments.