I think you've already answered it. (and you are overstating the direness)
in last 5 games the guy is 33.3%from 3. for the rest of the season he's about 10% higher. But the whole team has dropped these 5 games. Because the opponents are better.
He has stilled scored 16, 15, 17, 26, 21 over this 5 game span, and team is 4-1...
3s attempted per game is a pretty small statistical sample. 1 or 2 makes or misses can dramatically alter the % either way. his worst % over these games was 4-15 (26.7%) one more make and he's 33.3, 2 more and he's .400...
Like I said, over the last 5 he is 33% more or less. One more make per game (23-54 instead of 18-54) and he is right at 42.5%, or just about his season average minus these games. So he's basically missing one more per game...