I have no dust trouble with my Epson V850 holders - all three formats. I am kind of fastidious about my process. If the negs are not new, I clean them first with PEC-12, but not new ones - I have a built in hanging cabinet in the darkroom and take care in washing and hanging.
When scanning, I first open one track on the holder, run the orange anti-static cloth over the side which will face the neg base side, and also the strip of negs, then Rocket blast both. I place the neg strip in, then close the holding part.
I do this with all strips on that holder.
Then I wipe both glass surfaces in the scanner with the cloth, and the base side of the holder.
Then I rocket blast all the surfaces, including both pieces of glass.
What's important is wiping/blasting each surface just before it's ready to be positioned in its final place, the sequence is logical. It's time consuming, but much faster than spotting in PShop. I look for dust immediately after opening the scan, and find very few dust spots.
My issue on the holders is that (with Leica M cameras) it's very difficult to position the neg and reveal both sides of the rebate, one edge of the image is almost always cropped out. And with very even toned areas (like the sky on a clear day) there is a long area of slight overexposure parallel to the edge of the neg/holder, almost like a reflection from the edge of the plastic (and you can't see it in the preview). I can usually take it out in PShop with careful cloning/brushing, but it's a pain. It doesn't happen all the time, but sometimes I just tape the neg base side down to a piece of ANR glass, and space it up the the right distance above the glass, scan, and it disappears.
Anyone else have this problem? And it's not surge from development, as it doesn't appear when scanning a neg mounted to glass.