Wait, I'm an idiot ... the problem also stops when I stop rainmeter. My money is on rainmeter being the problem.

I'm at the end of my rope trying to fix this. When displayfusion is running my desktop wallpaper flickers between a solid black and the picture I have selected as background. I'm using an image that spans all monitors (although the same thing happens with assigning different images to monitors.) The issue stops when display fusion isn't running.

I have been in advanced settings and disabled some of the wallpaper "timers" in there with no effect. I've disabled wallpaper management all together. I've disabled lock screen wallpaper. I've restarted the app after all of these changes. Nothing seems to work.

I do have stardock curtains running as well. I initially thought that display fusion and curtains might be fighting over the wallpaper but stopping curtains does noting. Only stopping displayfusion seems to stop the flickering.

Just imagine the wallpaper image disappearing and there is just a solid black background. After ~2 seconds the wallpaper image returns. It does this every ~10-20 seconds or so ... I haven't timed it.

So it's probably rainmenter and displayfusion fighting over the wallpaper or something like that. Rainmeter can be a buggy mess so I think I'm going to scrap my rainmenter setup and reinstall it from scratch and see what that does.

So I did a fresh reinstall of rainmeter and attempted to set it up as I had previously. I had a very "busy" rainmeter layout. (I do network monitoring systems for a living, I like lots of gauges and graphs.) It went right back to the flashing wallpaper. Only happened when both rainmeter and displayfusion are running.
Scrapped the busy rainmeter layout for a different set of skins that are much simpler and haven't had an issue with that. So I'm pretty sure this is some kind of rainmeter issue that is just exacerbated by having displayfusion running. Rainmeter has always been a bit dodgy so I'm not sure why I didn't suspect it first.

I'm doing ok with the simpler setup.

If you really want to experiment ... when I say a busy rainmeter setup I mean something like this: https://www.deviantart.com/fonpaolo/art/AVENGERS-S-H-I-E-L-D-2-1-506438856