As you did not include any settings or a photo of how you take your images, I simply assume what could go wrong.

I assume that you are using you light panel without additional diffusion under your negatives holder and then have the camera on a tripod above the setup to take the image, resulting in a raw file.

Things to check:

• Are you using constant settings aka manual mode? All sources you photograph should be already correctly exposed, so adding any auto setting in here will result in a huge variation of exposures in your images. So once the exposure is set, it should the same for all images, unless you change anything in the setup. Valoi actually recommends dialing in the exposure at +0.7 to +1 EV as you might be losing some light through the image (and probably because, dimming an image will do better for the dark parts regarding signal to noise)
• Did you lock your color balance for the light you are using? Set light and camera to 5600k.
• Are you getting additional light from the panel into your lens? Shining additional light from around the frame holder into you lens might influence overall contrast by flaring.
• Have you made 100% sure (by taking a photo with the light panel switched off) that you are not getting ambient light in? If you are not getting a pitch black frame by this test, you are throwing the contrast off. If you are getting ambient light in, reduce ambient light. Switch off additional lamps, close curtains etc.
• Can you elaborate what programs you are using for reverting the negatives to positives? With color negatives, it is not enough to simply invert the image. The photographer you mention uses Negative Lab Pro.
• You might want to add some distance and additional diffusion to the light panel. A light panel with settable color temperature has at least 2 different color LEDs in the panel. Used as a light source, it evens out by distance. If you place something on the surface of the light, that might not be the case. You might end up with cooler and warmer colors identifiable in the shot.
• Are you using fully mechanical shutter? Sometimes LED panels are casing problems with the electronic shutter, resulting in banding. As you are using a video panel, this should not be the case, but you could test anyways just to make sure.