**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.
{
    for (const auto & disk : getDisks())
    {
        for (auto it = disk->iterateDirectory(relative_data_path); it->isValid(); it->next())
        {
            if (startsWith(it->name(), "mutation_"))
            {
                MergeTreeMutationEntry entry(disk, relative_data_path, it->name());
                UInt64 block_number = entry.block_number;
                LOG_DEBUG(log, "Loading mutation: {} entry, commands size: {}", it->name(), entry.commands.size());
                auto inserted = current_mutations_by_version.try_emplace(block_number, std::move(entry)).second;
                if (!inserted)
                    throw Exception(ErrorCodes::LOGICAL_ERROR, "Mutation {} already exists, it's a bug", block_number);
            }
            else if (startsWith(it->name(), "tmp_mutation_"))
            {
                disk->removeFile(it->path());
            }
        }
    }
    if (!current_mutations_by_version.empty())
        increment.value = std::max(increment.value.load(), current_mutations_by_version.rbegin()->first);
}
std::shared_ptr<MergeMutateSelectedEntry> StorageMergeTree::selectPartsToMerge(