Our policy is to enter the forward limit CoG and the forecast cruise temperature. This generates a conservative MAX ALT.
The problem of the MAX ALT dropping (sometimes considerably) can be due to the cruise temperature being warmer than expected. The FMC defaults to ISA. If nothing else has been entered, the FMC will calculate MAX ALT on a predicted cruise temperature of ISA. When cruise altitude is reached, the MAX ALT and amber bands are then calculated on ACTUAL temp.
Therefore, if the actual temperature is ISA +10 or 15, the MAX ALT will drop and the gap between the upper and lower amber bands will reduce.