are you sure that the test cases you print are those you fail ? I suspect that you were printing the island after having mutated it. I tried your code and it always fails the random tests in the same fashion: the starting point contains some gold ($) and it seems like you fail to collect it, so you have an extra step that you should not have:

start = (0, 0), steps = 2, expected = 0, actual = 9

$..
...
9..

I'm closing as the issue seems to be coming from your code. Feel free to present more information.

thanks for the quick fix

I've changed the order of the error message creation with the call to the user's function so it now shows the original value of the input.

Print the input at the start of your function, you're mutating it and that's why the error message seems weird.

...because other solutions don't have this problem, maybe there is something more specific

could you show me your solution, please?