This may be similar or related to MDL-31958? This is happening on 2.2.1+ (Build: 20120112)
Steps to recreate:
-Created a scorm activity and set it to "show activity complete when conditions are met" and check "Student must receive a grade to complete this activity". save and close
-In the settings block open "completion tracking" mark the scorm activity as a completion requirement. Also mark and set a grade as a second completion requirement.

• now go back into the scorm activity settings and change "show activity complete when conditions are met" to "do not indicate activity completion"

this is when the bug occurs. the scorm activity still appears as a completion requirement to students and is in the completion report but it never marks as complete. Open "completion tracking" and the scorm activity is unavailable but if you return the activity completion settings back to "show activity complete when conditions are met" and check "Student must receive a grade to complete this activity" the option to select the scorm activity in "completion tracking" reappears and is selected.

basically if start with the activities editing screen and change "show activity complete when conditions are met" to "do not indicate activity completion" without removing the requirement from "completion tracking" first, the bug occurs. If I remove the requirement from "completion tracking" then change "show activity complete when conditions are met" to "do not indicate activity completion" all is good.

Thank you for Moodle!