I've had this problem for quite some time now (for months, maybe even over a year), but I keep forgetting to report it when I have the time.
Occasionally, when I try to access my computer after a DisplayFusion-controlled screensaver is active, instead of what should happen, the screen saver turns off and all I see is a blank screen in the same color as my Windows taskbar color setting. The only way to get out of that blank screen is to press Ctrl+Alt+Del to pull up the screen that pops up in Windows 10 from doing that, and then navigating elsewhere from there.

I haven't really tested it with multiple screensavers, so I'm not sure if it's the particular one that I'm using that's causing the issue. It might be that the particular one I'm using is somehow conflicting with DisplayFusion's coding.
I do not have it set to lock my computer, though, so it's definite not that that's causing the issue.

So I noticed that my primary monitor was set to "Blank" as its screen saver and not "--No Screen Saver--", where as my other screens that don't have a screen saver have it set to the latter. Only one of my screens actually has a screen saver, and it's an uncommon one that came as a bonus from a Japanese release of a music CD (as strange as that may sound for such a thing to come with a screensaver).
Ever since I switched my primary monitor from "Blank" to "--No Screen Saver--" as well, I haven't encountered the issue yet. I'll let you know if I still have that issue if it happens again.

If you still want to test that screen saver, then I have uploaded the installer for it, although it's a pretty badly programmed installer and might look questionable to some people. So I understand if you don't end up testing it for that reason. I'm not even sure where to find the screen saver file, itself, after it gets installed in order to upload that file directly (or if that's even possible).

But if anything, maybe check on the "Blank" screensaver option that Windows has by default to see if it conflicts with the "--No Screen Saver--" selection in any way, as that has a chance of being the cause of the problem? Unless I'm just getting lucky recently and the rare glitch that I've been having hasn't come up yet for some reason.