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.

Why isn't [2,0,2,4] == [8,0,0,0]?

Really tricky and puzzling kata to solve I'd say, compliments, I had a lot of fun with it.

Your kata was.... Unexplainably low for 6 kyu.
Good Job on making me cry solving this :)

This Kata had me hooked on the game for a while. :-D
Damn. Never reached 2048. I gave up crying.

so hard for 6kyu, but so interesting

Approved.

Yes. When you don't know, please don't raise an issue. Also use markdown tags to format your code or it's not usable. Refer to the documentation about this and more: https://docs.codewars.com/training/troubleshooting/#post-discourse

Rust translation

No.

Ruby 3.0 should be enabled, see this to learn how to do it

Please organize the structure of test fixture as following (although it has been mentioned in the attached link, I'm repeated here again)

describe "<This message should describe the categories of test groups inside this block>" do
  it "<A short message describing this test group>" do
    expect(...).to eq(...) #Assertions
    #Or Test.assert_equals(user_response, reference_response)
  end
end