So first thing: you never want to edit etc/system/default/props.conf. You should only be making changes to etc/system/local/*.conf, or a default/local entry in a custom app.
Second thing: If it's coming from a remote Splunk Universal forwarder, it has already gone through the input queue and will hit the indexers at the parsing queue. This will not use those props.conf settings as INDEXED_EXTRACTIONS only works at the input queue. So you want to go to your Universal Forwarder where the json is ingested, and configure your props.conf there, in either a new app or etc/system/local/props.conf.
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