solution was probably valid but now does not work anymore with the added mutation test as remove() method mutates the original array/list

for a = 100 and b = 99, although a and b is true, your return says that's false

That would work. However, it is a solution inefficient in terms of memory, as you are creating another list having the same size of the original.
There are better ways, as seen in other people's solutions.

It also isn't currently accepted as a solution.

this mutates the original list

Technically correct; however, the original list was mutated. Try deep-copying the list that was passed in, then work with those values instead. :)