I came across this issue when trying to breed horses, so I set up blocks with snow layers to measure their jumping height. I noticed overtime these layers were melting somehow, so in an effort to remove all potential causes I moved it far away from anything, according to the wiki, only certain light blocks and a light level or a desert biome should be able to melt snow layers.

However, the effect still persisted. So I copied the world and tested in creative, making a small area of snow layers and waited. Overtime, the blocks started to melt despite no conditions being met. Unless i'm missing something, I believe this is a bug, my first theory is the light level from the sun is causing the melting, but there is no mention of this anywhere and i'm certain it hasn't been like this before.

In the attached screenshots, it shows the layer over time melting. The first being after I realised you wanted a screenshot, 2 snow layers had already melted. The second being halfway in-between the two. And the third after a frustrating period of time of trying to get OneDrive to work properly to give you high quality screenshots before giving up. Sorry for the poor quality.