I wonder if you may be having issues with Simulcast Distortion (
Simulcast digital distortion - The RadioReference Wiki), which often happens when a scanner gets strong signals from multiple towers in a simulcast radio system. This gets worse as a better antenna is used since it makes the signals stronger, which can lead to more distortion issues. One of the real problems with simulcast distortion in scanners is that it appears to be a lack of signal strength causing the issue, although it's probably actually an excess of signal strength.
Some scanners include an attenuator that can be activated to reduce the signal levels as a way to cope with the issue. You may try activating yours to see if that helps. Other things to trying a portable scanner antenna on your 996XT inside the vehicle to see what that does. With it being inside and rather well shielded you may reduce the signal enough to help. It may even be worth removing the antenna entirely (if you don't have access to a portable's antenna or get no help that way) to see how much signal you get that way. Some folks have resolved the issue that way.
The digital radio you got from the local PD was specifically designed to handle a simulcast system and to correctly process the signals to prevent them mixing and causing the distortion issues. This can be done in a high dollar radio but not a relatively inexpensive scanner simply because the high dollar radio can afford to put in the high dollar circuitry needed to perform this function while the scanner makers can't and still meet the design price points. It's also much easer to solve the issue with a single band radio (like your PD's radio) than a very wide band one (like a scanner) simply due to the need to only handle a small range of frequencies. The few multi-band commercial radios handle the issue as well, but often have seperate circuits for each band (which is one reason they're so expensive).