Frozen vegetables and pizza will generally remain safe to consume if the freezer temperature fluctuates from -18°C to -12°C or -14°C for a short period of time. While the ideal temperature for freezing food is -18°C or below, a temporary increase in temperature within the freezer range you mentioned is unlikely to cause spoilage or significant degradation of the food.
However, it's important to note that these temperature fluctuations can affect the quality of the frozen items over time. The longer the food is exposed to temperatures above the recommended freezing temperature, the more likely it is to experience changes in texture, flavor, and nutritional value. The freezing process helps to preserve the quality of the food, and any deviation from the ideal temperature can lead to some degradation.
To maintain the best quality, it's advisable to return the freezer to its optimal temperature range as soon as possible. Additionally, if you notice any visible signs of spoilage, such as ice crystals, freezer burn, or significant changes in texture or odor, it's best to discard the affected items to ensure food safety.