@EqualsThree(View Post) It looks like it is turning into snow, but it is hard to tell because the thermite is going so fast. I collected everything that was left over so that nothing was lost and all of the thermite was still there. The thing is that once it has cleared the snow, It speeds up again and reaches high speed... which means the thermite would be off the screen quickly.
What you're actually seeing is the thermite displacing the snow.
When thermite tries to move to a spot already occupied by snow, the thermite and the snow swap places. In this case, because it's moving so fast, the thermite's previous position is a large distance away from the block of snow - so it does look a bit odd when snow suddenly appears there.
Not exactly a bug, but not ideal behaviour either...