Poked at it more as a response to your question, and yes, I've found out what's causing it.
With the simple search "|metadata hosts" as an example:
With the visualization type set to "Events", the panel displays "Search did not return any events."
If you change the visualization type to "Statistics", it will happily display data as you'd expect.
I'm not sure of the reason why the output of |commands counts as "statistics" rather than "events",
but that would seem to be the case.
I'm going to put that down as "Confusing, but working as designed."
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