Why did Japan stop to play just before the game ends on a match vs Poland?

Poland was eliminated regardless, they wanted to go out with a win. The match between Senegal vs Colombia was happening at the same time with Senegal losing 0-1 as well and if result had stayed (0-1 in both match; Sengal vs Colombia and Poland vs Japan) Japan & Colombia would win the tie breakers and advance to the next stage.

If Senegal had scored a goal, Japan would've been eliminated. So it'd be better for Japan to at least draw with Poland as well in case Senegal also draw with Colombia in the other match but Japan didn't want to risk losing more and gambled that Colombia could hold the result.

In short, Japan gambled on the other match, not their own.

The Japan vs Poland game was happening at the same time as the Senegal vs Colombia game. At the end of the games Poland and Colombia were winning their games by one goal. Given the standings Colombia would go through as first seed with 6 points, while Japan and Senegal were tied with 4 points.

Japan and Senegal had the same number of points, goals, goal differential. But Japan held the tie breaker by having 2 less yellow cards than Senegal.

So instead of attacking in the stoppage time and push for a goal Japan took the safer way by keeping the ball. Had they pushed for a goal they would've been vulnerable to counter attacks, which could lead into goals for Poland or more yellow cards for Japan which would've reduce from their tie breaker over Senegal. Also Poland was already eliminated and were winning the game so they didn't need to push Japan to get the ball back. So Japan made sure to keep control of the ball without trying to score and hoped that on the other game Senegal wouldn't score a goal.

At the end the score of both games remained at 1-0. Japan made it to the round of 16 for having 2 less yellow cards based on fair play performance.

I didn't watch the game, but from what I've heard, the result of the Colombia-Senegal game meant that Japan were still set to qualify from the group even though they were losing 1-0. However, as they were tied with Senegal on points and goal difference, conceding another goal would knock Japan out and send Senegal through instead.

If Japan pushed forward in search of an equaliser, they risked being caught out by a counter-attack and conceding a second goal that would have eliminated them. All they needed to do for those final two minutes in order to qualify, was keep Poland from getting hold of the ball. And that's exactly what they did. A disappointing end to the game, perhaps, but they were thinking in the long-term.