Here's another theory, your framerate of 240fps may be suddenly falling to 60fps intermittently.
That generates 4x more motion blur,
http://www.testufo.com demonstrates this well, 240fps vs 120fps vs 60fps.
Are you
SURE you are running 240 frames per second?
Also, make sure to unplug (completely) all 60Hz monitors.
If you earn money in eSports -- or depend on ultra-competitive gameplay where milliseconds matter --
Never run multimonitor. Do not run a 240Hz monitor and a 60Hz monitor simultaneously. Sometimes the 60Hz secondary monitor injects lag problems into the 240Hz. And/or suddenly reducing framerates to 60 frames per second (4 times more motion blur).
The sudden appearance/disappearance of motion blur can be an undiagnosed framerate-halving or framerate-quartering problem, which is extremely common (unfortunately) on mixed-Hz multimonitor setups.
Motion blur is inversely proportional to framerate, as seen in this TestUFO motion test below:
On a 240Hz monitor, this shows up as 240fps+120fps+60fps in Chrome/FireFox browsers.
On a 120Hz monitor, this shows up as 120fps+60fps+30fps in Chrome/FireFox browsers.
On a 60Hz monitor, this shows up as 60fps+30fps+15fps in Chrome/FireFox browsers.
Persistence (motion blur) is determined by frame visibility time. Which is shortened by an increased number of images per second (aka refresh cycles per second, ala visible frames per second) on a sample-and-hold display. That said, when running at lower framerates (without framerate-vs-Hz aliasing effects -- e.g. integral divisor framerates or VRR) -- then at 60fps, your 240Hz monitor is essentially giving exactly the same persistence as 60fps at 60Hz -- on the same panel. 60fps@240Hz has identical motion blur as 60fps@60Hz on the same LCD panel tech. So if your 240fps suddenly instantly slows down to 120fps or 60Hz, you'll see sudden appearance of motion blur.
At 240Hz, you you will observe that the 240fps UFO has approximately one-quarter the motion blur of the 60fps UFO. This is the case at least for a good TN 240Hz monitor (one that does not have too many overdrive issues).
TL;DR: You said your motion blur suddenly appears/disappears mysteriously. The "sudden appearance" of motion blur is sometimes simply a framerate-halving or framerate-quartering problem caused by a multimonitor setup. This is not always the cause, but unfortunately common. There are other causes (which we'll still have to sleuth) if this is not the issue.
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Sleuth Attempt #1 -- Therefore, if your game is looking like 240fps UFO then sometimes suddenly look suddenly blurrier like the 60fps UFO sometimes.....
May I be Sherlock Holmes and ask, do you have a 60Hz monitor attached to your computer as a secondary monitor?. Bingo. Mystery solved. Unplug/disable it and your problem is solved.
Sleuth Attempt #2 -- If you are running only 1 monitor, then something else is causing a potential problem. Can you turn on the framerate display in Fortnite and watch it? Find out if the framerate is halving/quartering when the motion blur suddenly appears. Enable the "Show FPS" setting in "Video" tab of Fortnite. Report back on the changes in framerate you see when you see motion blur suddenly appear.
If you see wild fluctuations in framerate that aren't Hz-aliasing related (Mathematically, simple 113fps has twice the motion blur of 226fps -- so any sudden framerate halving of any kind -- will always double tracking-based motion blur on a non-strobed LCD display) ......then try turning off AA and slightly reducing detail level, then this may help stabilize the framerate.