Hi @GoodResource ,
Have you tried creating a DLP compliant test email and checking if this email is caught by DLP? Detailed steps for your reference: Test a DLP policy.
If mail is caught by DLP, it indicates that your DLP work well.
If the email cannot be caught by DLP, compare the migration steps in the official documentation to check if there are any missing steps.
If all of above don't work for you, you could try to recreate a DLP policy in M365 Compliance center to check if it works well.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.