After moving a flow from an update set and opening the flow record from the flow designer, it shows error message " Flow "sys_id" Not found" .
The main flow is having a sub flow and it is not captured in the update set which is committed to moving the main flow.
the sys_id in the error message corresponds to a [sys_hub_flow_snapshot] record, from the snapshot record you should be able to determine what subflow it refers to
Please capture the sub flow on a new update set and commit the update set. This should fix the issue and open the main flow without any errors