Isn't this a bug?

In this game, after I played the 54th move, my opponent played the 55th move, and I can't see where he played after completing a box. Even if he did play something after that (but I believe he didn't), there's a minor bug because the box closing move is highlighted, as if it was the last one played

This exactly happened to me too in this game.

I try restart it. There is a bug in mobile version of game.

Mobile version was updated and is in Apple and Google play store.

This just happened to my opponent in this game as well; they completed a box but didn't get an additional move:

https://www.littlegolem.net/jsp/game/game.jsp?gid=2543905&nmove=56

The bugged move in the game I posted was probably made before you submitted the app fix.

This one, too:

https://www.littlegolem.net/jsp/game/game.jsp?gid=2543895&nmove=57

Both of those games I mentioned were corrected. Thank you, Richard!