Because the goal of our function is to implement "one swipe"(if this explanation makes the goal clearer):

A given tile can only merge once. (this limits any tile to be summed/combined only 1 time)

Your solution is mutating the input array and as a consequence, the tests are misbehaving. It is indeed a kata issue because the tests should not be vulnerable to this, but at the same time your solution is not exactly correct:

This function takes the array line as a parameter and returns a new array with the tile values from line slid towards the front of the array (index 0) and merged.