bytes[0] = 255;

should be changed to

bytes[0] = 1;

then test will pass.

In C# underlying type for boolean is byte, you can't really make true out of 255. But you can do it out of 1.

Regarding Assert.IsTrue(). What nunit does in this case is something like this:
Under the hood nunit does this :
public override ConstraintResult ApplyTo(object actual)
{
return new ConstraintResult((IConstraint) this, actual, true.Equals(actual));
}

so it is basically an equals operator:
If you distill this part into code

bytes[0] = 255;
NativeArray<bool> bools = bytes.Reinterpret<bool>();
var b1 = bools[0].Equals(true); // false

bool[] pureBool = {true};
pureBool[0] = true;
var b2 = pureBool[0].Equals(true); // true

So equals ends up comparing 1 to 255

The reason why error message looks so weird, is that ToString method doesn't really know how to translates both 255 and 1 into True.