When there's a small portion of lava and you place snow on the lava, the snow doesn't melt. it does melt when there's a pool of lava, but even with a small portion of lava, the snow should always melt when interacting with lava.

How to reproduce

Start by obtaining a lava and snow bucket
Then place a small portion of lava on a small hole like shown in the video, then place the snow on the lava and it will not melt