I would insist on running it in Compatibility Mode... make it compatible with Windows 7, and try a different preview mode in Poser.  Also make sure Windows 11 is not running in a resolution higher than 1080p. Poser preview may not be able to adapt to any 2K or 4K resolutions your Windows is running on. That's why the window flashes, the Task Manager is trying to tell you: "Hold on, I'm not done doing whatever I was doing! I cannot take on any other task til this one is done."