As far as I can tell, microflows are not possible in an offline hybrid app. I receive the error message: "Microflows cannot be used on pages that are accessible through the offline profile."
Take a look at the documentation: https://docs.mendix.com/refguide/offline
As Robert says, it is not possible to directly invoke microflows from an offline app. What you can use however, are before/after commit microflows which will be triggered on synchronization, and receive updated objects back. You can even manually invoke synchronization (for example by using an action button or on save) to force this behavior.