**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.
const ASTPtr & new_settings,
        AlterLockHolder & )
{
    if (new_settings)
    {
        bool has_storage_policy_changed = false;
        const auto & new_changes = new_settings->as<const ASTSetQuery &>().changes;
        for (const auto & change : new_changes)
        {
            if (change.name == "storage_policy")
            {
                StoragePolicyPtr new_storage_policy = getContext()->getStoragePolicy(change.value.safeGet<String>());
                StoragePolicyPtr old_storage_policy = getStoragePolicy();
                
                if (new_storage_policy != old_storage_policy)
                {
                    checkStoragePolicy(new_storage_policy);
                    std::unordered_set<String> all_diff_disk_names;
                    for (const auto & disk : new_storage_policy->getDisks())
                        all_diff_disk_names.insert(disk->getName());
                    for (const auto & disk : old_storage_policy->getDisks())
                        all_diff_disk_names.erase(disk->getName());