Also had this issue (5.4.4, and still here we are), and, as well as someone said above - switching to non-virtual textures in material that outputs result to RVT helped.
Then, I found that if using vitrual textures in material, output becomes blurry when I manually change VT pool size in the project settings (Project Settings → Engine → Virtual Texture Pool), or sometimes if it changes automatically (although in most cases pool auto grow didn’t caused blurriness, but manual changes always caused it - either when I changed default pool size, or changed fixed/transient pools). Weird thing that if you just open texture assets used in material - brurriness goes away (while other things, even engine restart/recreating all setup from scratch didn’t helpedl).
So, after I set default pool size to a bigger value (default is 64mb which is really small, although for me changing it to 256 mb was enough to not see texture loads), and disable checkbox ‘Pool Auto Grow in Editor’ - with this I was able to get rid of blurriness and still use VTs in material… in PIE. Sadly, but when using Standalone - blurriness returns. So there is definetly smth wrong with Texture Sampler when using VT/or with VT itself, but I haven’t been able to figure out what exactly.