**How to reproduce**
Clickhouse server 20.13.1.5273
```
CREATE TABLE default.test
(
`key` UInt32,
`ts` DateTime CODEC(DoubleDelta, LZ4),
`db_time` DateTime DEFAULT now() COMMENT 'spout-ignore' CODEC(DoubleDelta, LZ4)
)
ENGINE = MergeTree
PARTITION BY toStartOfTenMinutes(db_time)
ORDER BY (key, ts)
TTL db_time + toIntervalHour(3)
SETTINGS index_granularity = 8192, merge_with_ttl_timeout = 3600, min_rows_for_compact_part = 1000000, min_bytes_for_compact_part = 200000000, in_memory_parts_enable_wal = 0;


INSERT INTO test(key, ts) SELECT number % 1000, now() + intDiv(number,1000) FROM numbers(500);

SELECT * FROM test;
500 rows in set. Elapsed: 0.003 sec.

DETACH TABLE test;

ATTACH TABLE test;

SELECT * FROM test;

0 rows in set. Elapsed: 0.002 sec.
```

**Expected behavior**
Clickhouse would flush parts on disk.
if (merger_mutator.merges_blocker.isCancelled())
            throw Exception("Cancelled merging parts", ErrorCodes::ABORTED);
        merge_mutate_entry = selectPartsToMerge(
            metadata_snapshot,
            aggressive,
            partition_id,
            final,
            out_disable_reason,
            table_lock_holder,
            lock,
            optimize_skip_merged_partitions,
            &select_decision);
    }
    
    if (select_decision == SelectPartsDecision::NOTHING_TO_MERGE)
        return true;
    if (!merge_mutate_entry)
        return false;
    
    auto task = std::make_shared<MergePlainMergeTreeTask>(
        *this, metadata_snapshot, deduplicate, deduplicate_by_columns, merge_mutate_entry, table_lock_holder, [](bool){});
    executeHere(task);
    return true;
}
bool StorageMergeTree::partIsAssignedToBackgroundOperation(const DataPartPtr & part) const
{
    std::lock_guard background_processing_lock(currently_processing_in_background_mutex);
    return currently_merging_mutating_parts.count(part);
}
std::shared_ptr<MergeMutateSelectedEntry> StorageMergeTree::selectPartsToMutate(