I know when I make mine, at the point of adding the butter the mixture will start to look a little soupy but after I've finished adding all the butter and I just let it whip for another couple of minutes it comes right back to the lovely silky buttercream that IMBC is.
Also, if the syrup hasn't cooled enough before you put the butter in it will go soupy. I noticed that you use corn syrup?? I don't use that in the recipe I make...I use Warren Brown's recipe and I learned to make IMBC from watching his youtube video.
I have heard others say that they have no luck making it but from the first time I made it after watching his video, I've had no problems. I will attach the link in case you might be interested in trying his method of making it.
I'm sorry I can't be of more help. Maybe someone else might have some suggestions.