Elasticsearch stops refreshing - elasticsearch

Question

in Elasticsearch 6.6.2 i'm having this weird problem
where elastic stops refreshing all indexes. Any UPDATE/INDEX/DELETE operation
returns OK result but search is not showing changes. I see no errors in logs.
If i try to manually force refresh by API this will run forever with no response.
If i restart ES it resumes working correctly and all documents indexed during blackout are there. The problem appears again after few days. This is happening on only one single instance out of many tens that we have in production with exactly the same configuration.
All indexes are open and green.
UPDATE:
this is the output of /someindex/_settings?include_defaults during problem:
{
"someindex": {
"settings": {
"index": {
"creation_date": "1627983403578",
"number_of_shards": "5",
"number_of_replicas": "1",
"uuid": "eWWHKAsHQYix3uZSTs6YXg",
"version": {
"created": "6060299"
},
"provided_name": "someindex"
}
},
"defaults": {
"index": {
"max_inner_result_window": "100",
"unassigned": {
"node_left": {
"delayed_timeout": "1m"
}
},
"max_terms_count": "65536",
"lifecycle": {
"name": "",
"rollover_alias": "",
"indexing_complete": "false"
},
"routing_partition_size": "1",
"max_docvalue_fields_search": "100",
"merge": {
"scheduler": {
"max_thread_count": "2",
"auto_throttle": "true",
"max_merge_count": "7"
},
"policy": {
"reclaim_deletes_weight": "2.0",
"floor_segment": "2mb",
"max_merge_at_once_explicit": "30",
"max_merge_at_once": "10",
"max_merged_segment": "5gb",
"expunge_deletes_allowed": "10.0",
"segments_per_tier": "10.0",
"deletes_pct_allowed": "33.0"
}
},
"max_refresh_listeners": "1000",
"max_regex_length": "1000",
"load_fixed_bitset_filters_eagerly": "true",
"number_of_routing_shards": "5",
"write": {
"wait_for_active_shards": "1"
},
"mapping": {
"coerce": "false",
"nested_fields": {
"limit": "50"
},
"depth": {
"limit": "20"
},
"ignore_malformed": "false",
"total_fields": {
"limit": "1000"
}
},
"source_only": "false",
"soft_deletes": {
"enabled": "false",
"retention": {
"operations": "0"
}
},
"max_script_fields": "32",
"query": {
"default_field": [
"*"
],
"parse": {
"allow_unmapped_fields": "true"
}
},
"format": "0",
"frozen": "false",
"sort": {
"missing": [],
"mode": [],
"field": [],
"order": []
},
"priority": "1",
"codec": "default",
"max_rescore_window": "10000",
"max_adjacency_matrix_filters": "100",
"gc_deletes": "60s",
"optimize_auto_generated_id": "true",
"max_ngram_diff": "1",
"translog": {
"generation_threshold_size": "64mb",
"flush_threshold_size": "512mb",
"sync_interval": "5s",
"retention": {
"size": "512mb",
"age": "12h"
},
"durability": "REQUEST"
},
"auto_expand_replicas": "false",
"mapper": {
"dynamic": "true"
},
"requests": {
"cache": {
"enable": "true"
}
},
"data_path": "",
"highlight": {
"max_analyzed_offset": "-1"
},
"routing": {
"rebalance": {
"enable": "all"
},
"allocation": {
"enable": "all",
"total_shards_per_node": "-1"
}
},
"search": {
"slowlog": {
"level": "TRACE",
"threshold": {
"fetch": {
"warn": "-1",
"trace": "-1",
"debug": "-1",
"info": "-1"
},
"query": {
"warn": "-1",
"trace": "-1",
"debug": "-1",
"info": "-1"
}
}
},
"throttled": "false"
},
"fielddata": {
"cache": "node"
},
"default_pipeline": "_none",
"max_slices_per_scroll": "1024",
"shard": {
"check_on_startup": "false"
},
"xpack": {
"watcher": {
"template": {
"version": ""
}
},
"version": "",
"ccr": {
"following_index": "false"
}
},
"percolator": {
"map_unmapped_fields_as_text": "false",
"map_unmapped_fields_as_string": "false"
},
"allocation": {
"max_retries": "5"
},
"refresh_interval": "1s",
"indexing": {
"slowlog": {
"reformat": "true",
"threshold": {
"index": {
"warn": "-1",
"trace": "-1",
"debug": "-1",
"info": "-1"
}
},
"source": "1000",
"level": "TRACE"
}
},
"compound_format": "0.1",
"blocks": {
"metadata": "false",
"read": "false",
"read_only_allow_delete": "false",
"read_only": "false",
"write": "false"
},
"max_result_window": "10000",
"store": {
"stats_refresh_interval": "10s",
"type": "",
"fs": {
"fs_lock": "native"
},
"preload": []
},
"queries": {
"cache": {
"enabled": "true"
}
},
"ttl": {
"disable_purge": "false"
},
"warmer": {
"enabled": "true"
},
"max_shingle_diff": "3",
"query_string": {
"lenient": "false"
}
}
}
}
}
Any hint?
Thanks