If I drag the TileMap to the left or right the number of tiles that hide the projectile increase.
There seems to be a vertical divider on the map that hides the projectile. If one tile hides the projectile then all of the other tiles above and below will also hide it.
Also, there are no 1/2 measures. The projectile could stick 1/2 out of the sprite, but once it crosses the threshold it is no longer visible.
Maybe it is a render culling of some sort?