Previously, a retention policy could be associated with both the custom object that you explicitly set it for and another custom object with a similar key prefix. This behavior no longer happens.
Where: This change applies to Lightning Experience, Salesforce Classic, and all versions of the mobile app in Enterprise, Performance, and Unlimited editions.
Who: Field Audit Trail is available to customers who purchased a Salesforce Shield or Salesforce Field Audit Trail add-on subscription.
How: A key prefix is the three-character code in an object ID that specifies the object type, such as 001 for Account. Let’s say you previously created two custom objects, Object1 and Object2, whose key prefixes differ only in their case sensitivity, such as a0B and a0b. You then defined a retention policy for Object1 but not Object2. Salesforce used to associate the same retention policy with Object2. Starting in this release, Salesforce no longer associates the policy with Object2.
Retention Policy Now Associated with Only One Custom Object (salesforce.com)