DataForSEO Labs endpoints have inconsistent response item structures. related_keywords nests data under keyword_data.keyword and keyword_data.keyword_info.search_volume, while keyword_suggestions uses a flat structure: item.keyword and item.keyword_info.search_volume directly on the item.
This causes silent zero-volume results if you copy the nested extraction pattern from related_keywords to keyword_suggestions — the keyword extracts as empty string and volume as 0, but no error is raised because .get() returns defaults.
Flat structure endpoints (verified): keyword_suggestions
Nested keyword_data structure endpoints (verified): related_keywords
The server-side filter (['keyword_info.search_volume', '>', N]) works on both — the API returns results matching the filter, but if you parse the wrong nesting level, you get the keywords with 0 volume.
Debug tip: print sorted(item.keys()) for the first item. Flat items have keyword and keyword_info at the top level. Nested items have keyword_data containing those fields.