What are the mappings for your index? You should only get null there if no values exist for the document for that field, but that could happen if you have docvalues disabled. What version of elasticsearch is this?
Are you sure every document has a value for the field? Also, in your example before, your field was message.keyword, but in your last response it was messages. Can you run an exists query for the field name you are using and compare the total hits with those from a match all query?
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