Just like mixing lemon juice with milk will cause curdling, mixing some naturally tart Monin Syrups with milk (especially hot milk) may not be recommended.
If a flavor is really tart in nature (e.g. lemon, lime, orange, pomegranate) the corresponding syrup will usually not be recommended with dairy products.
French Raspberry Syrup and Candied Orange Syrup are tart flavors that have been specially formulated to work well with dairy ingredients.
Try mixing 1/4 oz. of Vanilla or French Vanilla Syrup with the tart flavoring before adding any milk product. This sometimes works to balance the acidity level and prevent curdling from occurring, without changing the overall taste profile drastically.
For additional questions, feel free to contact us at support@help.monin.com or at 800-966-5225