I have reproduced your experience. The problem seems to be linked with blanks in the distractors as you suggested. As a result the correct answer was present in the HTML you could see but the selected answer did not have a 'selected' string to make it get displayed.
I have just tried a fix by doing a trim on the distractors and it seems to fix it. However I need to experiment in case there is some reason why leading blanks are necessary to preserve (I cannot think of any reason why that would be true)
For reference and not for live this is what I have just tried (once on a test machine that nobody uses). In the file questiontype.php
In the method get_answer_fields
Around line 354 I changed
$question->wronganswers['text'] = rtrim($question->wronganswers['text'], ',');
To read
$question->wronganswers['text'] = trim(rtrim($question->wronganswers['text'], ','));
Note the additional trim that trims blanks after the first call has trimmed trailing commas.
I will experiment, test and write a new phpunit test and come up with version 1.92 and post here when it is ready.