**How to reproduce**
Clickhouse version 20.11.5.18, 20.8.7.15, 20.3.19
```
SELECT
    groupUniqArray(val) AS uniq,
    toTypeName(uniq),
    groupArray(val) AS arr,
    toTypeName(arr)
FROM
(
    SELECT CAST(number % 2, 'Enum(\'hello\' = 1, \'world\' = 0)') AS val
    FROM numbers(2)
)

Query id: d4657f27-c8bd-4174-9947-c6c48d0222d5

┌─uniq──┬─toTypeName(groupUniqArray(val))─┬─arr───────────────┬─toTypeName(groupArray(val))────────────┐
│ [0,1] │ Array(Int8)                     │ ['world','hello'] │ Array(Enum8('world' = 0, 'hello' = 1)) │
└───────┴─────────────────────────────────┴───────────────────┴────────────────────────────────────────┘
```

{
        if (data(rhs).value.empty()) 
            return;
        auto & a = data(place);
        auto & b = data(rhs);
        if (b.total_values <= max_elems)
        {
            for (size_t i = 0; i < b.value.size(); ++i)
                insert(a, b.value[i], arena);
        }
        else if (a.total_values <= max_elems)
        {
            decltype(a.value) from;
            from.swap(a.value, arena);
            for (auto & node : b.value)
                a.value.push_back(node->clone(arena), arena);
            a.total_values = b.total_values;
            for (size_t i = 0; i < from.size(); ++i)
                insert(a, from[i], arena);