You should state more precisely your question. (for example, which UV filters are you using). Anyway, at first sight, a very old formulation is used as the base emulsion. All emulsions are unstable, as a principle. You should read about emulsions theory and the causes of instability. Use Stoke´s law as the first "rule of the thumb" to optimize your formulation. Failure in your test points to aggregation phenomena during the freezing cycle in your test. You have to understand the relationbetween micro properties of your system and the macro experimental results.

Emulsions are thermodynamically unstable systems that can be temporarily stabilized for the shelf life of the product. Elevated temperatures or freezing will in any case contribute to the destabilization of the system, more or less rapidly. Looking at your formula, your emulsifier is unique, glyceryl monostearate, which is not the most effective with a significant oily phase, considering not only paraffin oil but also stearic acid, cetyl alcohol, and vaseline. It would have been important to have the percentages of each ingredient to check the total oily phase percentage and the surfactant percentage. The emulsion is thickened by the presence of stearic acid, cetyl alcohol, and vaseline, which solidify at room temperature, contributing to its apparent stability. Freezing tends to crystallize most of the ingredients in the formulation. Furthermore, most stability tests for cosmetic or pharmaceutical formulations allow temperature variations without using extremes, which are not recommended or specified in the ICH guidelines.