We have implemented sv_min_team_size for a map to specify the minimum size of a team at finish to count as a team rank. All team ranks on 3-4 player maps with fewer players were deleted as team ranks, but stay as regular ranks. The motivation is not to ditch your partner in the middle of the map to get a better time. Thanks to @Rockus for the suggestion, @Kicker and @murpi for determining the maps in question.
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