I agree that it doesn't always increase as expected.
I'l have to keep a close eye on it because I'm pretty sure that it doesn't budge until you are levelling out and even then only 100ft.
In the cruise you are reducing weight by about 2200kg an hour and the optimum increases 100ft every 6 1/2 - 7 1/2 minutes.
In the climb, if it was using the same computational logic, it would increase 100ft every 2-3 minutes and over a 20 minute climb you would see close to 1000ft change and we would all be well aware of this.
I think that the update 10.8 might be different to the way it used to work but I'm not sure.
Start a timer and watch it on the climb out, I'l do the same. The surface temperatures are rarely high where I'm based but I'l keep an eye on that too.