Email messages that are older than 30 days are missing from the Deleted Items folder of a user's mailbox in your organization. This problem occurs even though the default email retention period for deleted items was extended in Exchange Online.
This problem can occur if one of the following conditions is true:
A custom retention policy was created and assigned to the mailbox.
The Default MRM Policy was renamed.
To increase the retention period for the Deleted items folder, do one of the following:
Increase the number of days in the custom retention policy.
Assign the Default MRM Policy to the mailbox.
Rename the retention policy that's assigned to the mailbox as "Default MRM Policy" (without the quotation marks).
In Exchange Online, the length of time that items in remain in the Deleted Items folder was extended from 30 days to indefinitely or to the duration that is set by the administrator. This retention period can be overridden either by renaming the Default MRM Policy or by assigning a custom retention policy to a mailbox.
For more information, see the following resources: