colour shift on b/w with adjustments

[DavidGB]

MacOS. 10.15.7     Affinity Publisher 1.10.4

I am preparing a book of black and white photos to be printed in four color B/W. The picture files are RGB placed into my Affinity doc set up in cmyk. Images which I adjust (levels, brightness/contrast etc.) in Affinity Publisher all show a colour shift. Most of the time the shift is very subtle and barely noticeable in Publisher but when I export to PDF the shift becomes very obvious. The greater the adjustment, the greater the colour shift usually towards blue. I have used the same method to publish full colour images with adjustments giving no adverse effect.

Is this a bug or am I doing something wrong?

See the attached screengrab

[lacerto]

Obsolete.

[DavidGB]

Thanks for that.  Recolour set to zero appears to work best.

[lacerto]

Obsolete.

[DavidGB]

This is all at the edge of my knowledge and experience. I will have to look at it more closely.

Thanks for your help.

[lacerto]

Obsolete.