Fixed it.
In my case, I had to make sure that on the Splunk Cloud instance the same sourcetype was defined and also had KV_MODE = none .
I had defined the type on my Universal Forwarder, but had not appreciated that some of the properties, like KV_MODE, are search time properties, and hence they would have to be defined on the search instance (not just the forwarded).
I didn't have to use the AUTO_KV_JSON = false setting in the end.
You put me on the right path though with the index vs search time double indexing - thanks!
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