Setting the "Build Periodically" setting for a multibranch pipeline job implies that the job will be run at that interval. But the setting is intended to trigger periodic rescans of the repo, not periodic builds.

The inline documentation implies that the build will be run periodically, as opposed to the branch indexing. This is misleading:

Provides a cron-like feature to periodically execute this project.
This feature is primarily for using Jenkins as a cron replacement, and it is not ideal for continuously building software projects. When people first start continuous integration, they are often so used to the idea of regularly scheduled builds like nightly/weekly that they use this feature. However, the point of continuous integration is to start a build as soon as a change is made, to provide a quick feedback to the change. To do that you need to hook up SCM change notification to Jenkins.
So, before using this feature, stop and ask yourself if this is really what you want.