When editing a List of custom objects as shown below, they get parsed correctly, but when editing them in the GUI config they are not saved.

private final List<PairOfInts> complexList = Arrays.asList(new PairOfInts(0, 1), new PairOfInts(3, 7));

private static class PairOfInts {
        private int foo;
        private int bar;

        PairOfInts() {
            this(1, 2);
        }

        PairOfInts(int foo, int bar) {
            this.foo = foo;
            this.bar = bar;
        }
    }

And if you changed it from the config file directly, it will erase the new value with default ones...
There is an example is the mod that does it (the exact same code shown here) and it doesn't work so I don't get why it's in example. Really sad it doesn't work...