Inconsistencies in response for the same query (when server overloaded)

[Petit-Benjamin]

Describe the bug
I'm currently trying to determine to optimal throughput of our Pelias server. While modifying the amount (and speed) of requests I send to the service, I faced some inconsistencies in the responses (for same queries). This happens only when the server returned errors 500 during the process (because it has been overloaded).
What is interesting is that the queries for which I don't have the same response are not the ones that faced the error 500, but some others.

Is there something in the search algorithm that would impact the results when the server is getting overloaded?

Expected behavior
I would expect a query to always returns the same result, even if the server is overloaded

Pastebin/Screenshots
There is an example of different results for a same query

{'geocoding': {'version': '0.2', 'query': {'parsed_text': {'city': 'baku', 'postalcode': 'AZ1010', 'country': 'Azerbaijan', 'housenumber': '3rd floor', 'street': 'Landmark II, 45a Khagani Street'}, 'size': 5, 'private': False, 'boundary.country': ['AZE'], 'lang': {'name': 'English', 'iso6391': 'en', 'iso6393': 'eng', 'via': 'default', 'defaulted': True}, 'querySize': 20}, 'engine': {'name': 'Pelias', 'author': 'Mapzen', 'version': '1.0'}, }, 'type': 'FeatureCollection', 'features': [{'type': 'Feature', 'geometry': {'type': 'Point', 'coordinates': [49.840288, 40.378395]}, 'properties': {'id': '421174967', 'gid': 'whosonfirst:locality:421174967', 'layer': 'locality', 'source': 'whosonfirst', 'source_id': '421174967', 'country_code': 'AZ', 'name': 'Baku', 'confidence': 0.6, 'match_type': 'fallback', 'accuracy': 'centroid', 'country': 'Azerbaijan', 'country_gid': 'whosonfirst:country:85632717', 'country_a': 'AZE', 'macroregion': 'Absheron', 'macroregion_gid': 'whosonfirst:macroregion:1108808621', 'region': 'Baki', 'region_gid': 'whosonfirst:region:85668249', 'region_a': 'BA', 'locality': 'Baku', 'locality_gid': 'whosonfirst:locality:421174967', 'continent': 'Asia', 'continent_gid': 'whosonfirst:continent:102191569', 'label': 'Baku, BA, Azerbaijan', 'addendum': {'concordances': {'gn:id': 587084, 'gp:id': 1951874, 'loc:id': 'n79110308', 'ne:id': 1159151123, 'qs_pg:id': 911331, 'wd:id': 'Q9248'}}}, 'bbox': [49.544202, 40.307031, 50.075926, 40.496085]}], 'bbox': [49.544202, 40.307031, 50.075926, 40.496085]}

{'geocoding': {'version': '0.2', 'query': {'parsed_text': {'city': 'baku', 'postalcode': 'AZ1010', 'country': 'Azerbaijan', 'housenumber': '3rd floor', 'street': 'Landmark II, 45a Khagani Street'}, 'size': 5, 'private': False, 'boundary.country': ['AZE'], 'lang': {'name': 'English', 'iso6391': 'en', 'iso6393': 'eng', 'via': 'default', 'defaulted': True}, 'querySize': 20}, 'engine': {'name': 'Pelias', 'author': 'Mapzen', 'version': '1.0'}, }, 'type': 'FeatureCollection', 'features': [{'type': 'Feature', 'geometry': {'type': 'Point', 'coordinates': [49.89201, 40.37767]}, 'properties': {'id': '587084', 'gid': 'geonames:locality:587084', 'layer': 'locality', 'source': 'geonames', 'source_id': '587084', 'country_code': 'AZ', 'name': 'Baku', 'confidence': 0.6, 'match_type': 'fallback', 'accuracy': 'centroid', 'country': 'Azerbaijan', 'country_gid': 'whosonfirst:country:85632717', 'country_a': 'AZE', 'macroregion': 'Absheron', 'macroregion_gid': 'whosonfirst:macroregion:1108808621', 'region': 'Baki', 'region_gid': 'whosonfirst:region:85668249', 'region_a': 'BA', 'locality': 'Baku', 'locality_gid': 'geonames:locality:587084', 'continent': 'Asia', 'continent_gid': 'whosonfirst:continent:102191569', 'label': 'Baku, BA, Azerbaijan', 'addendum': {'geonames': {'feature_code': 'PPLC'}}}}], 'bbox': [49.89201, 40.37767, 49.89201, 40.37767]}