Hi, this is a known issue.
Forest computes a different set of items, depending if current Max state is "viewport" or "rendering". For example for viewport, numbers of items is limited, and for render Point-cloud is not needed and skipped.
This is a safe procedure and follows the rules defined in the Max SDK, because in Max both states (viewport and render) cannot co-exist.
But interactive renders (both V-Ray and Corona), uses their own method which mixes viewport and render modes. This is not a standard Max procedure, and causes several undesired side effects, specially with complex plugins as FP or RailClone.
Briefly, i want to say this problem is not simply a Forest bug, but something more difficult to solve.
Said that, we already fixed this limitation in Forest Pack 6 (actually in development), creating two separated distributions sets, for viewport and render.
We have to rewrite several parts of the code, which must be tested carefully. For this reason, we cannot make the changes in FP5 easily.
We are on the final development stage of FP6, and i hope the first beta is available in a couple of months.