I received a reply from Splunk Support. They said so far it appears to be a benign message -- the 'runshellscript' actually does complete. The message only occurs on HFs and can be eliminated by setting precalculate_required_fields_for_alerts = 0 in your /local/savedsearches.conf file -- add to the stanza for the saved search associated with your 'run a script' trigger action. They DID mention this is safe if no other alert actions are attached to that search. Basically, the required fields logic is in place for all alert actions but is no use in this one, so the logic is not needed. However, if other alert actions are attached to the search, you could encounter a performance impact. I just decided to create a custom alert action, so I haven't tested this workaround and I cannot confirm that it works but perhaps this could help to fix your issues.
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