Datehistogram returning key_as_string in different formats between different clusters?

ES version 5.6.3 / 5.6.4
I have an index with a date field of format "strict_date_time_no_millis||strict_date_time"
When I do a date histogram in 5.6.4 it returns the key_as_string using strict_date_time_no_millis format, when I query 5.6.3 it returns it in strict_date_time format.
How is the format chosen for the key_as_string?
Is it possible the formats are in a map and it's random on server startup as to which format is chosen?
Or was there a change in 5.6.4. I didn't see anything in the changelog for it.

I'm going to try an upgrade of 5.6.3 -> 5.6.4 now, I'm just wondering if there was a change, or whether this will be a random issue?

Upgraded to 5.6.4 and I still have the same issue.
One machine is running Windows 10 and the other Ubuntu, maybe that could be a difference.

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