Anyone had problems, when setting transparent background to Snackbar?

That seems to be the shadow drawn by the elevation property. Have you tried setting the elevation to 0?

yeah, just wanted to write that 🙂

Elevation doesn't work well with translucent surfaces

Good solution is to use your color with alpha composed over surface color:

MaterialTheme.colors.onSurface.copy(alpha = 0.12f).compositeOver(color = MaterialTheme.colors.surface)

This way you get the same effect of alpha color but you can have elevation