I have eaten pineapple and milk together many times and nothing happened with me.
It is often seen that different places have different beliefs about food. These beliefs have been prevalent since a very long time and people might have believed in those beliefs due to lack of complete information at that time, but now things have changed a lot.
Even today I see that the old people around us are very serious about some old beliefs and follow those things regularly. Whereas if seen scientifically, those things have no importance now.