The related fields specified in the trigger condition of a survey is not populated with any values on the "Assessment Instance" table.
a) There is a public survey which means it can be taken by guest users without really logged in the system
b) Trigger condition has some related fields configured like related field 1 "assigned_to" and related field2 is "assignment_group"
User should have access to view the records in order to get related fields populated on assessment instance record. If the survey is taken by guest won't have related fields populated.
This behavior can be reproducible on the baseline instance (London/Madrid). This is not a product defect.