XnView Software

When I open a jpegli-XYB its colours are correct
If I use Image - Adjust - Curves to change the brightness it does so wrongly, creating a green image if a luminance curve is dragged low
However if I first copy what's open and paste to a new image, then the curves are applied correctly
This is surprising given you'd think Curves would work on the straight image that's there in both cases
Let me know if any problem reproducing

I just tested this and can confirm the same issue — the green tint appears when adjusting the luminance curve directly. Copy–paste to a new image fixes it for me too. Definitely seems like a bug.

I do like the XYB profile, so I'd be keen to hear from xnview that all the colour-bugs I've mentioned (curves, paste, especially lossless crop) will aim to be fixed since I've started converting my camera images to XYB format and will feel relaxed if there's a clear plan to see it sorted. My approach is to save images as jpegli XYB then transcode them losslessly to .jxl which makes them smaller still but provides the reversibility that they can be losslessly transcoded back to jpeg when needed. In fact it would be rather nice if XNView provided this as an option for jpeg ("then transcode to jxl effort N")

yes JPEG-li need to be loaded as RGB (XYB profile applied)
Thanks to your detailed description I can reproduce the problem.