The Altas server status metrics show that roughly 4% of multiplanner invocations have fewer than 2 plan candidates:

https://docs.google.com/document/d/1jCfwdOIEHb3EAAtaThk1PTu7Q7-7SqE0buqEJMovFYM/edit?tab=t.0#bookmark=id.e7mppekk12g1 

Queries with one candidate plan should not be multiplanned, so there is either a multi-planner-enablement-logic bug or a bug in the counter. Such a bug would have no measurable impact on performance, since winning plans are resumed where they left off in the trial phase, but this should still be cleaned up.

For the metric being tracked, see “classicNumPlansHistogram” in multi_plan.cpp.

This could also be a data pipeline issue (in the translation from server metric to the data warehouse).