The only possible plot hole is...

THE RULE: The first person you see after you travel in time is the ONLY person who can see you. Nobody else can see you except that person.

THE POSSIBLE PLOT HOLE: When Xianglun travelled back in time in the end, the first person he saw was Xiaoyu. Now, how was it possible for Xianglun to stay and live in the past when only Xiaoyu can see him? The class picture is a proof that everybody else can see him.

Or maybe, it has a different rule when one travels back in the PAST. The rule applied to Xiaoyu because she went to the FUTURE.

Great movie by the way. But it doesn't come close to Korea's IL MARE. That movie was perfectly made. I've watched that film several times and no matter how many times you try to find a hole in the plot, you'll never find one.