Fiddler,
Had some time to dig into this a bit more, here's a bit of a workaround that might work for you too.

I'm using Nvidia GTX970 graphics card, outputting to 2 Vizio 43" TVs through HDMI, both at 3840x2160/60p

(trying to add files, not sure how that will come out!)
In my Nvidia control panel, I have a setting to "Adjust desktop size and position" and it allows me to choose "Perform scaling on" where I can choose GPU or Display. I'm using (forcing) the GPU to do scaling, rather than the Display. Works great.

In my Windows Display Settings, I normally have both set to 150% scale, and under Advanced Scaling settings, I turned ON the blur setting (Let Windows try to fix apps so they're not blurry) Custom Scaling is left blank, it affects all displays, might be useful if you have some oddball percentage, which I'm not doing.

If you go a little further, at the very bottom of the Display settings page, there's a link to "Graphics settings".
Here, you can choose to customize performance (GPU again) for either "Classic app" or "Universal app" (STore apps!) I left mine on Classic app. So far, so good, everything works fine, except small Menu text on apps.

The workaround, is to set my secondary display scale setting to 200%, and leave the Primary display scaled at 150%.
You can do this "on the fly", it takes effect right away, and BIAB is scaled very nicely, no blur, and easier on the eyes! LOL

I have several video editors, where I prefer to have both displays scaled the same amount, to 150%, so I just switch it back, and works fine. WIth a single monitor, just keep in mind that if you change scaling, it'll affect all programs, which may or may not be an issue. Hope this helps!

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Jerry
2019 BiaB Megapack (634), RB 2019 (Build 4)
Reaper 5.983 x64, Audient id14 USB interface, Voicemeeter Potato 3.0.0.8, Nektar LX49+, Roland E20
i7-6700K Skylake desktop, 64bit Win10 Pro(1903), 32GB ram, 512GB system SSD + 2TB data SSD