Profile in settings.xml does not activate modules

I have a multi-module setup where one "sandbox" module should only be built in development and not on the CI server, so I moved that module to a profile stanza in the parent POM. I also added that sandbox profile to my .m2/settings.xml, active by default.

When I run any goals (including help:active-profiles, dependency:resolve, and compile), the sandbox profile is listed as active (source: external), but the reactor does not include the module in the build. If I explicitly specify the goal on the command line using -P sandbox, the module is correctly included.

(This behavior is visible on 3.1.1. I can't upgrade to 3.2 yet because of incompatibilities with Jenkins, and none of the issues listed as fixed in the 3.2 series matched the behavior.)