Sage Stallone's cause of death is still a mystery ... this according to his official death certificate filed in L.A.
The death certificate lists Sage's C.O.D. as "deferred" -- which means the coroner is waiting for the results of various tests, including toxicology, before proclaiming what killed the 36-year-old.
The coroner notes that Sage's body was found on July 13 -- but does not specify a definitive date of death.
As we previously reported, people connected with the case believe Sage may have been dead for several days before his body was discovered by the housekeeper.