XPATH is creating a WRONG SQL for Retrieve

See the xpath in a Retreive:
...
and 
  (
    ($MendixCID = empty or ($MendixCID != empty and Registration.Registration_Provider_Implant/Registration.Provider/UID = $MendixCID))
    or
    ($MendixCID = empty or ($MendixCID != empty and Registration.Registration_Provider_Following/Registration.Provider/UID = $MendixCID))
  ) 
] 

======== Defect in Mendix version 6.8.1 =====
My table has 2 records.
Refer to the "or"  in above xpath. If I remove any one of the 2 condition that are separated by "or" then both record satisfy the xpath and both are fetched.
But when I retain it as above i.e. 2 conditions separated by "or", then only 1 recorded is fetched.
How in the world can a "or" fetch less records???
$MendixCID is empty.

I know of ways to change the above xpath and make it work. But my question is why the above simple looking xpath generates wrong SQL.