Foliage does not paint on actors that have "auto generate collision" turned off during import.
Regression (yes) issue (does not) occur in 4.12.5
Foliage will paint over the static mesh from the browser, but paint through the mesh imported without "auto generate collision".
Foliage paints on both, as the line trace for foliage placement checks for visibility, not collision.
Head over to the existing Questions & Answers thread and let us know what's up.