C4DtoA 2.6.1 [4186e70b]
Arnold core 18.104.22.168
I can only get this problem to occur when the render region in the IPR is rendered in about less than half of one second. To try and reproduce this: new scene. cube primitive. add Arnold sky - physical sky mode. Set your render settings to 720p and leave arnold samples at default so that everything renders super fast. Start the IPR. Now create a render region that is over the rendered pixels of the cube. Make the render region around 20x20 pixels - so pretty darn small. (Perhaps there should be an indicator of some sort to tell you what your render region dimensions are. that would be really handy.) Zoom in to the render region (in the IPR) so that you can see what you're doing. Now drag the render region in any direction some small distance (just don't drag it off the cube). At some point / about 1/8th of the time, the region will fail to display the final progressive pass of the render. This is with your mouse still hovering over the center of the render region where the drag tool is. When your render region does fail to render this last pass, move your mouse off the render region and you will see that it displays the last pass finally.
There have been other cases where this problem occurs much more frequently but I can't figure out what the frequency depends on.
If you can't reproduce this issue then tell me so and I'll make a video and post it back here.