You need to edit the Data Loss Prevention Policy to disable the email notifications.
1. Go to https://protection.office.com or navigate to the Security & Compliance admin center.
2. In the left navigation pane, expand Data Loss Protection and select Policy.
3. Select the Data Loss Prevention policy and click the Edit Policy button.
4. Click Policy Settings in the left navigation pane of the policy.
5. Select the policy rule and click the Edit Rule button.
6. Scroll down to the 'User notifications' section.
7. Toggle the slider labelled "Use Notifications to inform users...." to Off.
8. Click Save to save the changes to the policy rule.
9. Click Save to save the changes to the policy.

The question states that "all the user account synchronizations completed successfully". Therefore, the synchronization rule is configured correctly. It is likely that the 10 user accounts are being excluded from the synchronization cycle by a filtering rule.
Reference:
https://docs.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-sync-configure-filtering

Debra will need the Mailbox Import Export and Mail Recipients roles to be able to import PST files. These roles cannot be assigned directly to a user account. The way to assign just those two roles to a user is to create a new role group, assign the roles to the role group and add the user as a member.
1. Go to the Exchange admin center.
2. Select Permissions.
3. In the Admin roles section, click the plus (+) sign to create a new role.
4. Give the role group a name such as PST Import.
5. In the roles section, click the plus (+) sign.
6. Select the Mailbox Import Export and Mail Recipients roles and click Add to add the roles.
7. In the Members section, click the plus (+) sign.
8. Select Debra Berger then click Add then Ok to add Debra as a member of the new role group.
9. Click the Save button to save the new role group.

Create an activity alert
1. Go to https://protection.office.com/managealerts.
2. Sign in to Office 365 using your work or school account.
3. On the Activity alerts page, click + New. The flyout page to create an activity alert is displayed.

4. Complete the following fields to create an activity alert:
Name - Type a name for the alert. Alert names must be unique within your organization.
Description (Optional) - Describe the alert, such as the activities and users being tracked, and the users that email notifications are sent to. Descriptions provide a quick and easy way to describe the purpose of the alert to other admins.
Alert type - Make sure the Custom option is selected.
Send this alert when - Click Send this alert when and then configure these two fields:
- Activities - Click the drop-down list to display the activities that you can create an alert for. This is the same activities list that's displayed when you search the Office 365 audit log. You can select one or more specific activities or you can click the activity group name to select all activities in the group. For a description of these activities, see the "Audited activities" section in Search the audit log. When a user performs any of the activities that you've added to the alert, an email notification is sent.
- Users - Click this box and then select one or more users. If the users in this box perform the activities that you added to the Activities box, an alert will be sent. Leave the Users box blank to send an alert when any user in your organization performs the activities specified by the alert.
Send this alert to - Click Send this alert, and then click in the Recipients box and type a name to add a users who will receive an email notification when a user (specified in the Users box) performs an activity (specified in the Activities box). Note that you are added to the list of recipients by default. You can remove your name from this list.
5. Click Save to create the alert. The new alert is displayed in the list on the Activity alerts page.

The status of the alert is set to On. Note that the recipients who will received an email notification when an alert is sent are also listed.
References:
https://docs.microsoft.com/en-us/microsoft-365/compliance/create-activity-alerts?view=o365-worldwide