Hello,
I don't know if this one has already been reported but in case this was not I'll take time to explain the bug I found.
I created 2 splines and scattered the same stone along the 2 splines.
I've set the proximity mask of the stone on the first spline to exclude some grass, it's working fine, but when I try to exclude the foliage from the second spline it doesn't work, even if both scattered stone appear in the grass proximity settings (see Image1).
I noticed both scatter actors had the same as I scattered the same object on both spline, so I tried to rename one of them and boom, it works! (see Image2).
Hope this "diag" may help you understand what's wrong under the hood and this workaround may help other people facing the same issue until it's fixed.
Anyway, you're doing an amazing job with Dash, I've been using it for a couple of days and this is so fun to use and a real game changer. Good job and cheers