Hi Vincent,
this also seems to be the case when querying the Labels via the CampaignService and/or the AdGroupService (API version 201705).
When querying the LABEL_REPORT we receive non-negative label ids, but we cannot use them for filtering eg the CAMPAIGN_PERFORMANCE_REPORT. However, we can use the negative label ids from the CampaignService as filters.
We rely on labels to avoid creating duplicate keywords so this is an important issue for us.
Regards,
Peter