Is it expected behavior to have font and halo thickness change when switching between RGB and CMYK color models for a layout? The issue seems to only be with labels that have a halo applied to them. I tried using labels with no halos and I did not see any significant difference in the appearance of those labels.
I have attached examples of what I am seeing. RGB produces labels where the font is thicker and the halo thinner, whereas CMYK produces labels where the font is thinner and the halo thicker. I thought the color model only affected the colors?
I've tried quite few variables here and still can't repro the problem. Would it be possible for you to send your repro case (for example a ppkx that contains a layout illustrating the probem) to https://support.esri.com/en/report-bug so we can start an investigation?