Incubation:APM DataDog endpoint tagset grouping bug
The DataDog series endpoint is not grouping data together via equivalent "tag set", aka a set of tags that is identical for a particular series item.
Data is being grouped by timestamp, host and measurement. This is fine for things like CPU where there is no tag set, or items where the tag set is always the same, but for disk usage where there is a specific "device" tag being set the order of the series data means that only the last grouping will survive, losing all prior devices.