You have to provide the following information whenever possible.

**Describe the bug**
This query fails with the latest stable version 21.5.5.12. It worked with the last stable version 21.4.7.3.

**Does it reproduce on recent release?**
yes

**How to reproduce**
21.5.5.12

```
SELECT
    label,
    number
FROM
(
    SELECT
        'a' AS label,
        number
    FROM
    (
        SELECT number
        FROM numbers(10)
    )
    UNION ALL
    SELECT
        'b' AS label,
        number
    FROM
    (
        SELECT number
        FROM numbers(10)
    )
)
WHERE number IN
(
    SELECT number
    FROM numbers(5)
)
```

**Expected behavior**
Query should run without errors

**Error message and/or stacktrace**
```
Received exception from server (version 21.5.5):
Code: 49. DB::Exception: Received from localhost:9000. DB::Exception: Block structure mismatch in Pipe::unitePipes stream: different names of columns:
label String String(size = 0), number UInt64 UInt64(size = 0), 'b' String Const(size = 0, String(size = 1))
label String String(size = 0), number UInt64 UInt64(size = 0), 'a' String Const(size = 0, String(size = 1)).
```

: header(checkHeaders(input_streams_))
    , max_threads(max_threads_)
{
    input_streams = std::move(input_streams_);
    if (input_streams.size() == 1)
        output_stream = input_streams.front();
    else
        output_stream = DataStream{.header = header};
}
QueryPipelinePtr UnionStep::updatePipeline(QueryPipelines pipelines, const BuildQueryPipelineSettings &)
{
    auto pipeline = std::make_unique<QueryPipeline>();
    QueryPipelineProcessorsCollector collector(*pipeline, this);
    if (pipelines.empty())
    {
        pipeline->init(Pipe(std::make_shared<NullSource>(output_stream->header)));
        processors = collector.detachProcessors();
        return pipeline;
    }
    *pipeline = QueryPipeline::unitePipelines(std::move(pipelines), max_threads);
    processors = collector.detachProcessors();
    return pipeline;
}
void UnionStep::describePipeline(FormatSettings & settings) const
{
    IQueryPlanStep::describePipeline(processors, settings);
}
}