It looks like it's returning documents that do not have these fields and thus the _source is empty.
Rather than using the match_all, you should use an exists query , so the search hits only the docs that do have these fields.
In my case, only some docs have the temp_cpu field and the first query (match_all) returned docs without this field, thus causing the empty _source. While the second one only hit the docs that have this field.
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