When user send email with attachment to their service-now instance, it triggers two inbound email actions 'Inbound Action 1' and 'Inbound Action 2' to create two records in different table. Ideally, both generated records should contain the attachment that sent from original sender. However, attachment exists in the record created by 'Inbound Action 1', but empty in the record created by 'Inbound Action 2'.
If we disable one of the inbound action like 'Inbound Action 1' , and send the same testing email with attachment. Inbound action 'Inbound Action 2' will create the record with attachment as expected.
This behaviour is expected and briefly documented here under "Attachment" heading:
"If an inbound email contains one or more email attachments, the inbound email action adds the attachments to the first record the action produces."
Attachments store the relationship to target record and can only be associated to one record at a time.