After migration from Plesk 9.5 to Plesk 11.5 if Customers transition scheme 'Upgrade to Reseller' was applied where the domain administrator account is exists but disabled, the domain becomes associated with a client of the reseller account instead of just assigning the domain to reseller.
But according to transition scheme explanation, correct behavior is as follows:
If domain administrator is disabled, subscription should be assigned to the reseller, customer should not be created. Domain/subscription to be owned by the reseller account
This is Plesk internal issue #PPPM-1675 , which has been fixed in Plesk 12.0 microupdate #7 .
Apply the latest Plesk microupdates .