This problem is caused by some odd routing in place with customers using Office 365.
When the digest is sent out, it's sent direct to the customers exchange without filtering it. This is because the digest contains details of quarantine/spam mail, such as the message subject, which could trigger a false positive.
When a customer is using Office 365 the digest gets routed back through us which is then causing it to be quarantined.
Our engineering team are currently investigating this issue, this however looks to be an Office365 routing problem.
As a workaround you could set up a filter at company level with the criteria below to stop the Quarantine Digests from being quarantined:
If Sender Address is firstname.lastname@example.org
Do Allow (skipping spam filter)
The engineering team are looking at this issue to determine if anything can be done about this.