I opened an Adobe support ticket. They confirmed that this is a bug in
the Adobe software. In fact, they reproduced the bug in AEM 6.3 SP1.
They have it logged as NPR-23821 (that's their Bugzilla/JIRA number). I
told Adobe that this isn't a critical issue for us, since we have
workarounds. Our workarounds are:1) Users can proactively share their
work queues without a need to initiate a request workflow2) An Admin can
manage queue sharing using adminui