Nikon NEF thumbnails showing too dark in Manage Workspace

[Bluesbirdy]

Hi, starting with the PSP2021 program for the first time I encounter a strange thing when I use the 'Manage' Workspace.
When I select a folder with my Nikon NEF photo's inside, the thumbnails are initially showing bright (like they should ...).
But then, after a second or so, a correction of some sort is being made and all the thumbnails and the preview image change into dull/darker images.
Has anyone an idea what the automatic correction is, that is being made?
And how to change that, if possible at all?
I tried a lot of things, but it didn't change anything.

BTW, I have read the post "Noise appears in preview image" by IronTim, but my problem has nothing to do with noise/high ISO settings.
Images are just changing after a few seconds, like a dark filter is applied. (RAW correction??)

Thanks.