Here I am rendering a crashing helicopter. I render the pyro explosion and rbd helicopter in different layer with force matte, but when I composite then in Nuke, there is a gap among the edge of the alpha matte, as seen in the picture.
When I tried to render them together in one layer, the problem don't exist.
I have tried shrinking or expanding alpha channel in Nuke, but it seems not the cause.

Doesn't anyone know something about this issue? I am using Houdini 17/17.5. Thx!