How were the graveyard spiral and other sensory illusions discovered?
This paper from the website of the Department of Behavioral Sciences and Leadership at the US Air Force Academy reviewed cases dating back to 1913 (note though it doesn't assert that the reports in 1913 suspected Spatial Disorientation (SD)). In the linked paper the author references a previous paper which I can't seem to find: Gibb RW . Historical assessment of visual spatial disorientation
[abstract] . Aviat Space Environ Med 2010 ; 81 : 318 . 


  In 2010, the lead author presented an assessment of visual spatial
  disorientation at the annual Aerospace Medical Association conference
  and cited 25 studies dating from 1947 to the present declaring SD’s
  role in mishaps as well as surveys of pilots anonymously sharing their
  SD experiences


The earliest research they reference is a Navy survey from 1947. Quoted from the linked paper


  1947: U.S. Naval aviators, 67 total, reported on their illusionary
  experiences and categorized them into visual, non-visual, conflicting
  sensory cues, dissociative, and emotional ( 48 ). Of note within
  visual illusions categories were confusion with lights, depth
  perception, “ black night, ” and judging height above the
  ground/water. Experienced pilots were still prone to illusions
  regardless of total flight time. 
  
  ( 48 )Vinacke WE . Illusions experienced by aircraft pilots while
  flying .J Aviat Med 1947 ; 18 : 308 – 25 .


The linked article suggests that the aviation community has known about these issues since the early days of flight but the issue was (and they assert still is) an underrated and underreported flight risk which may skew reporting which causes underfunding. Thus the published research may lag the time in which the community started noticing SD.