After investigation, I found out the task in the database.
Issue: This task is not finished, as the status is "starting" in the following image.(in Fig.1) If it is finished the status should be "succeeded" (in Fig. 2)
Reason: The issue with the problem has an empty positive prompt. Then AI did not know what to draw.
Solution: This is not a software bug in the backend. It is a feature for frontend, please don't allow users to submit empty strings, and if the status is not "succeeded" tell the user that this task is not successful.
Fig 1. task of the issue: nd9547dq2drgm0cg12k91rtxc8
After investigation, I found out the task in the database.