Yeah, this is actually a pretty common issue with some of the Negative Supply light sources. Seems to be hit-or-miss depending on the unit. I’ve run across posts here and elsewhere where people had the same kind of uneven backlight you’re seeing.
I couldn’t tell you exactly which models are most affected (I never kept close track), but a quick search pulls up plenty of examples. The usual fix seems to be either swapping for a better sample with more even lighting, or in some cases switching to another brand.
examples:
https://forums.negativelabpro.com/t/think-twice-before-buying-negative-supplys-scanning-kit/7912
https://www.reddit.com/r/largeformat/comments/18jkrcv/uneven_illlumination_from_negative_supply_4x5/