Empty vs. black background (New CC 2018 issue)

Hi all,

I'm a video producer for online courses and I'm coming across an issue I believe started after the CC 2018 release. I have footage of a lecture filmed in front of a black background but he was centered in the frame when it was filmed.  In order to add accompanying graphics I shifted him off to the side and used Lumetri to match the black background with the empty space now appearing on the side. I did all of this a year ago and there were no issues with this method. 

Now that I'm revisiting the footage and making small edits for a rerun of this course, I notice an issue with how I composited the sequence.  Now anytime another layer fades on top of this composition (in or out), the edge between the black background and the empty space to the side becomes very apparent.  At first I thought this was just an issue with the playback and would be fine upon export, but that appears to not be the case (other weird happenings occur during playback in that empty space, but never appear during export). Here's an image of a graphic fading out on top of the footage, exposing the edge noticeably (more so in this image than it does in the actual video).

I tried placing a black matte behind the footage, then I tried nesting them together, then I tried nesting the graphics on top, all to no avail.  I eventually figured out a workaround of placing a black matte on top of the footage, cropping it to the exposed edge and using a heavy feather to smooth it out. To a knowing eye you can still spot a bit of edge but it's quite acceptable.

Either way this used to not be a problem and now it is.  I would appreciate any help as to what is happening and why, and most importantly a less duct-taped method of fixing it.

Thanks!