Siemens has similar diagram as Rosemount does with an X through the roof center location.

Here's Vega's explanation:

Excepting conical vessels, the best practice for mounting a radar sensor at the top of a tank is to install it 200-500mm from a vessel wall. The reason for this suggested method is proven and scientifically sound: Microwave signals don’t necessarily reflect from a product in a straight line like a racquetball hit square against a flat wall. Because of the width of the path they travel, signals are more likely—especially in solids applications—to scatter, particularly in tanks with domed roofs where the problem is exacerbated. In any case, signal scattering is troublesome for through-air radar that converts time of flight to level. When signals pinball from the product around the vessel back to the product and eventually return to the sensor’s antenna, the sensor thinks the level is lower based on the signal’s time of flight. For this reason, instrumentation manufacturers have long cautioned against mounting a sensor in the center of a tank roof. It’s in the manual and everything.

https://www.vega.com/en/home_aw/Company/Blog/2017/...