why does tuna in olive oil and cooking cream often go wrong

When mixing tuna (from a can and heated with a little olive oil) with cooking cream (panna), often the whole thing goes wrong, unfortunately:

Instead of having a nice homogenized creamy sauce, the cream flocks-out (becomes cottage cheese like) and the tuna swims in greasy oil.

Why does this happen? It never occurs, when I heat chopped tomatoes together with the tuna and add the cream.