Isse with applying Layer Effects

[OrangeNebula]

First time posting after being a happy gimp user for many years

I am working on a project that looks way different in the editor form the export (see attached file).
I have seen posts before about colour modes when exporting but this is so drastic I am pretty sure something else is wrong.

I seem to have traced the issue back to a layer with a contrast effect added  non-destructively:
But when merging layers it looses this contrast, creating the lighter export picture rather than what I see in my editor now.

Is this a known bug? Am I being stupid? How do I get the image I see in my editor out?
Thank you guys in advance!

[rich2005]

@OrangeNebula Looking at your screenshot it looks like many layers including a layer group containing that layer effect. Plus the obvious dithering, what format is the image, RGB or Indexed ? Both open and when exported.

Helpful if you can post the affected file as a Gimp .xcf or maybe give a more complete screenshot of the layer dock. The Brightness-Contrast layer effect can be applied as previous Gimp by ticking the "apply" option in the tool dialogue (or merging in the layer dock). Does that make any difference ?