Since we moved ddmatch from YUI2 to YUI3, users are experimenting some problems.
See Don Hinkelman report in CONTRIB-4117:

I experienced three problems with ddmatch in 2.4 that I did not experience in 2.3.
1. incomplete colors (red and green) surrounding the feedback answers (minor)
2. text that is input in the boxes shows up (unreadable) in vertical columns when displayed (major)
3. when additional matches on the right are added to give more possible options, the additional match with no matching box shows up blank (major)

See also this forum thread:
https://moodle.org/mod/forum/discuss.php?d=226179#p982309
And the screenshots that Don sent to me that I will add to the present issue