EV 12.5 items showing as 'ineligible' for archiving

Thankyou both for your thoughts.

It turns out that the information I was working with was out-dated and the version of Exchange is 2019 and not 2016 as I thought. This then led me to the following technote that looks to be the root cause of the ineligible items and growing mailbox sizes.

Enterprise Vault for Exchange: in rare circumstances attachments and mail body are not stripped after the item archival (veritas.com)

We are in the process of upgrading to resolve this issue

many thanks