I have Kiwi 2.5.6, MixinBootstrap 1.0.3 and SnowRealMagic 1.7.5 with forge 31.2.0 installed on a completely clean 1.15.2 installation and when I try putting more than the base layer of snow on a block (fence, stairs, walls) just does nothing. I've also tried dropping multiple layers on a block, and they just disappear upon impact.

Is this on purpose, am I missing a config or what?

https://imgur.com/a/Gu994EF

It is on purpose. Stackable snow layer will occupy too many block states

Then please update the gif on the curseforge page, that would be nice.

That is feature for 1.12.2