Hi,
Disclaimer: First of all, I’m not pretending to be a super-duper smart (I’m not even using use the word “clever” here) person when it comes to Blender, and maybe my solution is just a workaround and not the right solution at the same time, but been hanging around with Blender for some time I did find a way to get rid of those weird jittered/grainy shadows in so-called new Eevee Next render.
Anyway, here’s what you would need to do in order to increase your shadow quality in Blender 4.3 upwards in Eevee Next, and at the same time, get rid of those grainy weird shadow effects (or at least make them seen less noticeable to an eye).
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Check “Jittered Shadows” (step No. 1) right under Sampling → Viewport settings in Render Properties tab as seen in the screenshot No. 1.

Screenshot No. 1 – Render Properties tab
1.1 Increase the number of your samples to something like 128 or more for both Viewport/Render (step No. 2 and 2.1).
1.2 In the Shadows subtab, increase your Resolution value all the way up to 1.000 and check Volume Shadows option by increasing its steps all the way up to 16 as well (step No. 3) -
Select your Light object in your Outliner tab (see screenshot No. 2), then switch to the Lighting tab (see screenshot No. 2.1),

Screenshot No. 2 – Outliner tab

2.1 Screenshot No. 2.1 – Light object settings
There, reduce the Radius value of your Light object to something like 0.001 (you can play with it in order to get a more suitable result), then check the “Soft Falloff” option as well, if it hasn’t been checked before. -
Here’s the final result you might get after all these tricks set and done:

Screenshot No. 3 – The final result
P. S. Thanks to @SeraphDesign777, we have found out that it also needs to turn on the Jitter Overblur option, and increase its value all the way up to 20% (some say it should stay at 0% to get better result, so you can play with this option as well), as shown in the screenshot below:
Screenshot No. 4 – The Jitter Overblur option
P. S. S. Actually, what I’ve written here was already covered in this one topic, but I thought that it’d be helpful to share it here with people, so others who run into this issue won’t have to dig through tons of threads to find the solution. Also, I have fixed some little issues here and there in order to make it even more better and understandable
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Hope that helps,
Thanks, and have fun!

