O no so how it works is that if the mob goes to activate the trash mechanic and it fails, it will store it otherwise it won’t be stored and it will just double tap you. This is one of those old style mechanic’s that was removed later in the game because it was no longer needed. Think of the thrash being on a chance on action. Mob goes to auto you, you are to far away then thrash proc’s but you are to far away so it becomes a stored attack. It’s just weird I would agree but it is kind of funny to watch people get thrashed by Firemaw.
The main reason was to back then prevent some of the cheesy things we do now but since this is classic and not vanilla it doesn’t matter because you can still play around it the vast majority of the time. For example, back then the original intent was to prevent people from skipping over other mechanics by having a secondary punishment mechanic.