Review the manage levels for the applicable account resource. The inheritance settings for "Unmanaged" and "Full Managed" will determine the retention of group membership when an account is disabled. These are available on the "Set manage levels" tab.
It's also important to review the inheritance settings on the "General" tab (of the account resource) that determine the behaviour for permanently and temporarily disabled accounts:
For more information regarding manage levels and account resources, please refer to the Identity Management guide, Manage Levels for Handling Active Directory User Accounts