Whenever I update a field extraction, both from the search head UI field extraction helper, and via props.conf or transforms.conf, it always takes several minutes before the changes take effect.
Some possibly relevant details:
Search head clustering in use
props.conf/transforms.conf changes applied from a search head deployer node, and pushed via
splunk apply shcluster-bundle.
Don't know direct answer to your question. I do know that adding | extract reload=true to your searches will force reload at search time which is helpful if the problem statement is troubleshooting field extractions.