This is a full board JSON export.
I did some trials around this :
1) exported JSON of the card only (and not the whole board). In this case I see the missing comment. This is to verify there is no issue inside the comment that interfere with the JSON creation.
2) I changed the location of the problematic card in the kanaban page. The same comments were missing in the JSON file.
I debug the JSON file with firefox as a viewer.
I must emphasize: the comments show on the board. They are not lost .
Would appreciate any solution.