This may happen only if you haven't completed the whole flow. In order for Discovery outcome to be updated, and lead moved to the next stage, you'd need to complete the whole Discovery call outcome flow.
For example, if a lead screening was successful after a discovery call, you need to:
Click on "Add outcome" button
Select Approved
Select new Strategy call event, that needs to be booked with the lead
Select event host:
Via default event host setup → Round Robin
or manually selecting a specific host of the event previously selected
By clicking "Schedule call" you will be redirected to the booking page, from which you will be able to select date and time of the Strategy call
Only after the new call is scheduled, status will change to Strategy call booked