Here is a screenshot of the standard Render Settings used and you can see the Rendered windows on the Right. The Grey Boxes are ones where I hit Cancel and it did not keep the partial rendered screen. I have not used 'rendering in a separate' I have not used 'render in background'

Now I have tried various Render Settings but was not monitoring This issue, just playing and experimenting. Also, there are a few times I did get the partial render, there are a few in the Render Window, but very few and they seem to be ones where I let the render go to near end before hitting Cancel.

In P11 it always kept the render regardless how soon I hit Cancel.... I do not consider this a HUGE issue, sometimes all those partials are actually a pain to clean up....but there are times I like to have them when working with lighting or effects for comparisons.