Hi John,
When you say "Since we've migrated a few users to Office 365", could you provide more information about the current situation? Did you migrate these users using Staged migration or deploy a hybrid Exchange environment?
And I want to confirm whether "Moderation for messages enabled" is the moderated transport feature in Exchange server.
As far as I know, in Exchange Online there is no moderated transport feature. The only related feature is the moderated recipient.
Regarding how to configure a moderated recipient in Exchange Online, you can refer to this article: Configure a Moderated Recipient in Exchange Online
Thanks,
Bourne Zhang