Thanks for writing to us.
- In case QA is unable to find RCA, then we should keep an eye on the application logs which can help Dev team to fix the issue.
- Also, We can evaluate the user data which is throwing the respective error. so that we can investigate if the issue is reproducible for particular data only.
- Further, As a good QA practice we must report such defects in the tracking tool and mention the frequency of occurrence(For Example : 1 out of 10 times). So that these issues are discussed in Scrum meeting and best could be done at right time.
I hope I answered your question.