This might not be an issue with Poser. Upon completing the render, Poser saves these renders to its own RenderCache folder, even if you don't save the files yourself. As an aside, there is an option in Poser under General Preferences > Render > Max cached renders that lets you control how many are saved.

Back to the point, since your scenes seem to render just fine but then disappear the moment they should be being saved to that RenderCache, it might be the case that your Windows 10 security settings are not allowing Poser to write to its save location (AppData in previous versions, seems to be in My Documents now under Poser > RenderCache). It might be worth checking your Windows notifications to see if it has blocked Poser from accessing these locations, or check 'Controlled folder access' via the Windows 10 Settings options for any warnings about Poser attempting (and failing) to gain access to these folders.