Funny - after you replied, I googled it. I think it is the kiwi. See this snippet from an article I found online.
Raw pineapple contains bromelain, an enzyme that breaks down the protein in your milk, making it bitter. The same principle is applied when bromelain is used as meat tenderizer, either as powdered additive or as fruit-based brine.
This process takes a short while, so if you used pineapple in your cereal and eat it right away , it will not be bitter (unless the enzyme content in your fruit is excepitonally high), but after 30 minutes it will be noticeable.
Bromelaine will also keeping gelatine from firming up. Heating destroys the enzyme, so canned pineapple is perfectly “safe”.
Other fruit with similar properties are papaya (papain) and kiwifruit (actinidin).