Interaction between metoprolol tartrate and lactose monohydrate

Has anyone seen brown spots on metoprolol tartrate tablets in the stability chamber of 75%RH and 40 degree celcius? I guess it might be due to the lactose monohydrate we use in our formulation as an excipient which is uptaking moisture, but my colleagues think it is due to the reaction between lactose monohydrate and metoprolol tartrate.
what are your opinions?
I appreciate your helps in advance.