If you're relying on the Render Cache as a "save" then you absolutely are losing renders. The Render Cache (Recent renders) is a CACHE. It only keeps the most recent renders. After a number of renders, set in General Preferences, the oldest renders are removed. You have to deliberately save the image.

Right click the image in recent renders and choose "Export Image"

(As with all things Poser there's a bunch of ways to export the image. This is just one of them)

Only then is the image truly saved.