I can confirm the effect you are seeing when Denoise AI (3.3.0) is used as a plugin from Luminar 4. The image returned into Luminar 4 going by the histogram appears to have had had both the lows and highs moved closer to clipping.
(In the comparison shot below you need to click on the image to see the right hand side of the image - I haven’t been able to constrain it so it all shows at the one time)
But that is not happening with the same test images when Denoise AI is used here as a plugin from Affinity Photo, or as a plugin from ADCSee Ultimate, both the 2021 and 2022 versions, so it appears to be an issue unique to Luminar 4. Something you could perhaps take up with Skylum.
Edit. A thought has occurred to me.
As I understand it, Luminar 4 is not fully color managed and simply uses the color profile set in the OS, presumably the one for the monitor (if indeed one has been set). I wonder whether that might have some bearing on this issue.