If DBClientBase initiates a listCollections command that gets interrupted, it swallows the Interrupt error code and uasserts with a non-specific error code (18630).

https://github.com/mongodb/mongo/blob/ac2279fdb62d64555e55077ba1113940a24afb04/src/mongo/client/dbclient_base.cpp#L583

The map-reduce operation (and possibly others) calls this function, and an interrupt during that call prevents the map-reduce from returning an error code that will identify the interrupt to the caller.

The swallowed error requires additional checking in my test, which needs to distinguish (expected) "Interrupted" failures from other kinds of failures.