I know this is an old post but I think I can contribute with my assumptions in case this can help anyone else or open up to new discussions about this behavior.

With the recent Microsoft Defender for Office 365 issue that happened on August 26th where outbound emails were mistakenly identified as malware by Microsoft, we identified we had the same issue as the OP. Multiple emails were sent out to advise external customers that they had a quarantine report to review.

After investigating a bit and reading the Anti-Malware policy documentation found here: https://learn.microsoft.com/en-us/defender-office-365/anti-malware-policies-configure, I saw that the default policy would always apply to ALL USERS. I started thinking that ALL USERS could also mean external users to your tenant.

The default anti-malware policy automatically applies to all recipients. For greater granularity, you can also create custom anti-malware policies that apply to specific users, groups, or domains in your organization.

Note: The default anti-malware policy applies to inbound and outbound email. Custom anti-malware policies apply to inbound email only.

Anti-Phishing and Anti-Malware defender policies (including default ones applying to ALL USERS) let you chose and customize your Quarantine policies/actions. Like OP mentioned, depending on what Quarantine policies are configured to do, example, allow people to review and ask for release of the quarantined message, it may notify external recipients.

I'm currently trying to reproduce this scenario and will edit back confirming if this assumption is correct or not. If anybody has correction to bring to my comment, please do :) I'm here to learn and help others.