When I transferred a card from one board to another it loses all the comments made up to that point. It still shows "7 comments were made on this card" in the icon at the top but the only comment that actually shows is the comment that it was transferred from another board.
In ACTIVITY you can find "X transferred this card from XXX". Click on XXX to read the original comments.
It is not comfortable, but it is the only way I found to have a look at these lost comments.