Since "Move to Maintenance" is a user action where the host should be evacuated we should trigger migrations also for VMs set up with "Manual" migration mode. Such VMs are migrateable one by one when specifically requested to migrate, moving to maintenance satisfies this requirement.
This bugzilla is included in oVirt 4.3.0 release, published on February 4th 2019. Since the problem described in this bug report should be resolved in oVirt 4.3.0 release, it has been closed with a resolution of CURRENT RELEASE. If the solution does not work for you, please open a new bug report.