Has an NPB (Japan) game ever ended 13-8?
It has. 13-8 has been a NPB (Japan) final 30 times in 63,173 games. The first was Chunichi Dragons beat Taiyo Whales on April 18, 1959; the latest was Saitama Seibu Lions beat Tohoku Rakuten Golden Eagles on August 14, 2020.
That makes 13-8 the 104th most common final score out of 223 that have ever occurred in NPB (Japan). One point away, 14-8 has happened 22 times and 13-9 has happened 24 times.