When Bluetooth signal is reflected by a surface inside the first Fresnel zone, it arrives at the receiver out of phase with the direct Bluetooth signal and reducing the signal amplitude. If there is no obstruction inside 60% of Fresnel Clearance Zone radius, signal reduction is small.
Fresnel Zone radius and clearance for Bluetooth signal for various distance is listed below. When Bluetooth range was 10 meters, multiple path interference was not an issue. It is the major factor in limiting range of long range Bluetooth 5 transmission.