According to the documentation, a fuzzy rectangle shape often occurs when the object is not correctly assigned to the desired layer. Instead, it can appear as part of the underlying layer (such as the first layer), resulting in a less defined, "fuzzy" visual appearance. Check the layer assignments and rule selectors for your PCB outline, ensuring it’s set as part of the top copper or designated board outline layer rather than a general background object.
For more specific guidance, please refer to the Selector-Based Layout Rules section in the Flux documentation.