On analyzing the reported behavior in our source level, we could able to fix to remain the popup open on mouse leave. But on fixing it, we could not able to close the submenu of RibbonMenuItem which has been opened at the same time. Since we could not fix this case ,as the items involved are framework elements. We decided to maintain the behavior of RibbonMenuItems which get closed on mouse leave.
Please let us know if you have any other concerns on this.