I'm fairly certain that back in the 2.0 days, SE claimed that all platforms would begin culling players when the density got to be around 200 in an 8 yalm radius or something like that. Chocobos contributed heavily to the problem, and bosses would usually be among the first things to vanish from the screen. SE eventually added priority culling so that the large scale fate bosses and your party members would always remain on screen. Now we're in Eureka with a limit of 144 players per instance, and we've gone backwards 3 steps with culling at lower numbers of sprites, bosses and party members vanishing. I'm unsure if spell effects go into the sprite count or not, but that might be a setting that can help?