Setting a biome's temperature does not properly control its snow line. 
In 1.17 and earlier, setting a biome's temperature adjusted it's snowline accordingly. Setting a temperature to 0.1495 for instance would result in patchy snow from around y80 down towards sea level, with full snow above 80. In 1.18 there is a jump of snow line from sea level at temperatures below 0.15 directly to y80 at 0.15. However, at 0.1499 the snow covers all surfaces from sea level to y79, then has a few blocks where the snow becomes patchy from y80 to y86, then goes back to full snow.