Both snow layers and snow blocks cause grass blocks beneath to appear snowy. However, in the case of the snow block, this effect is temporary - eventually the grass block will revert to dirt and destroy the transition.

It would be expected that this is due to the snow block being solid and blocking light, but snow layers as of 1.14 also block light, so I'm less sure that this behaviour is intentional in modern versions.

The fix could be the same as for MC-205060 - as well as making the block below appear snowy, blocks in the tag could also be made to not kill grass if within the tag.