I think that's happening because when you're increasing value intensity, pixels that didn't reach value of 1 yet, are treated by the different set of rules than pixels that are over 1 already. In other words, while pixel's value is increasing in 0-1 range, its represented IOR is decreasing and approaching 1,0, but as soon as that pixel's intensity goes beyond 1, its represented IOR starts increasing and approaching max value, i.e. IOR 999 I always though that this behaviour is global and uniform per map, but it looks that it's being calculated per pixel. I know it sounds complicated and i might be completely wrong, so it would be nice to get confirmation (or denial) from the team.