Once in a while, a multiple dropdown response is not graded correctly. See the image. Answer 3 shows what was selected, what the correct answer is, and is marked wrong even though it should be marked correct. Is there something that I should be doing to make this work correctly every time?
In new quizzes, if you are using a word bank with the drop-downs, make sure you do not repeat the choices from each drop down. The word bank lets students click and drag any response from any drop down into any blank. However, if you repeat options in multiple drop downs, those options will be in the bank more than once. If the correct answer for blank 1 is an incorrect answer for blank 2, students can then put the incorrect blank 2 option (identical to the correct blank 1 option) into blank 1. Unfortunately, canvas will mark that identical-to-the-correct response incorrect. So don't repeat options in a word bank.
If that does not explain this circumstance, it should be submitted in a canvas support ticket, so they can identify and fix the problem