There probably aren't many of us who would automatically expect to find chilli jam on a bacon a brie sandwich, but I wouldn't be at all surprised to find cranberry sauce. I guess it was this particular chef's take on his style of bacon & brie. Although, as the OP has stated, there was no menu available at the time, as they were behind getting them out.
It's one thing to expect a restaurant to list what's in a sandwich, on a menu, but what about more complicated dishes? You only have to google a recipe to see that there are many different variations of the same dish. Some may have 5 ingredients, whilst others may have 10, or far more, for the same dish. I think it unrealistic to expect restaurants/pubs to list every single ingredient, for every single dish, from appetisers, to main courses, and desserts.
Hence the reason why, anyone with an allergy, intolerance, or dislike, for a particular food, should alert the staff/chef. And, as many posters have already stated, restaurants are now becoming more aware of the importance of food allergies, etc., and well trained staff tend to ask when you order, but if they don't you need to make them aware.
Were I in that position, I would see it as my responsibility to alert the staff/chef, as I realise that it is not practical for every single food ingredient, for every single dish, to be listed on their menus. That is not me being argumentative, or siding with the staff/chef, over the customer. That is me being reasonable, and taking responsibility for myself, and not expecting someone else to be responsible for me.
It really isn't difficult to do that, and far better than leaving things to chance.