The run-as feature can only be used with selected item SharePoint flows, button flows and Dynamics 365 flows, but not with HTTP request flows. This is a real limitation:
1.if we want nested flows; indeed a good practice in general is to create resusable workflows and to do so in Microsoft Flow we have to implement HTTP request based flows that should run with the user context: but today if the caller flow is a run as flow starting with the for a selected item action, the sub flows cannot get the context.
2. we can create (sharepoint) custom action flows (without the for a selected item action) visible in the SharePoint list, but they cannot be shared with lambas users, only with owners. That is annoying.