Unable to use custom string representation in GString template

class JsonListAdapter implements List {
    private final List<Object> adaptee;
    private final JsonNode node;

    public JsonListAdapter(List<Object> adaptee, JsonNode node) {
        this.adaptee = adaptee;
        this.node = node;
    }

    // methods here

    @Override
    public String toString() {
        return node.toString();
    }
}

var json = "[\"v1\",\"v2\"]";
var node = parseJson(json);
var list = toList(node);
var result = gStringTemplateService.render("${variable}", Map.of("variable", new JsonListAdapter(list, node)));

// both assertions fail for now
Assertions.assertNotEquals("[v1, v2]", result);
Assertions.assertEquals(json, result);