maxTermsPerSegment

Distinct terms one index may hold for one segment.

Exceeding it drops the index for that segment, which then reads as not covered and is scanned. It does not keep the first n terms: a dictionary holding some of a path's values and no record of which ones is an index that returns wrong answers, and nothing downstream could tell it apart from a complete one. The same rule that makes a missing sidecar read as "not collected" rather than "collected and empty".