For every submission, the grading system reports the score for each subtask. If a subtask is not fully solved, the grading system gives a feedback only for the first test case among the lowest scored test cases in the subtask.
(Apparently CMS did not support the above rule for several years without anyone noticing)
This is to implement the following rule: https://ioi2021.sg/rules/#feedback, second paragraph:
(Apparently CMS did not support the above rule for several years without anyone noticing)