My apologies, seems I got marked as read while checking on mobile, and obviously forgot to reply.

As of your case, in vary rare case pool is unable to identify finder for certain block(could be various reasons, wallet RPC gets stuck, some occasional restart is needed then) in such cases we consider block as mine in shared mode and distribute proportionally.

The same applies always for merged mining coins - those are always distributed proportionally to hash contribtion regardless of solo or non solo mode.

cheers!
pinpin