I was doing some testing recently and I was able to use a Machine Flow in a Include Workflow in another Machine Flow.
i.e. Machine Flow A has include workflow step that uses Machine Flow B.
This seems to be working. Is there any issues with doing it like this? Or is it better to move the Machine Flow B content to a Workflow Fragment that can be used by both Machine Flow A and B?
Ops, I think you might have found something that shouldn’t be there. Not entirely sure, but I would recommend using fragments, as that is how it is meant to be (Included workflow - fragments | Flow Help).
As far as I know, there is no official support for using a machine flow as an included workflow so that it might yield unexpected behaviour. (Although the meta difference between a fragment and machine flow is pretty slight, they are not the same.)