Have you tried using the Queue Manager? My understanding of render in background (and I could be wrong) is that it is a poor man's way of getting a Local Queue Manager. Back when there was a Pro Version if you had Pro then you can rendered and keep using Poser, if you didn't then you used Render in Background. Basically the same thing except with Queue Manager you can also render on other computers. And there were all types of other reasons to do a background render such as the 4GB memory issue. But since Poser 12 only runs on Windows 10 which is 64 bit that reason is no longer valid.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13