I can't work out how that happened either. To see what is really going on here, it would be helpful to see what is going on behind the scenes. Are you able to get the output of a database query like this one for this question attempt?
https://docs.moodle.org/dev/Overview_of_the_Moodle_question_engine#Detailed_data_about_an_attempt
(Make sure you obscure any personal information before posting here. You can email the data to me if that seems more appropriate.)
https://docs.moodle.org/dev/Overview_of_the_Moodle_question_engine#Detailed_data_about_an_attempt
(Make sure you obscure any personal information before posting here. You can email the data to me if that seems more appropriate.)