Dolby Pro Logic IIx is a Dolby technology that expands stereo (2-channel) or 5.1 surround sound sources into 6.1 or 7.1 channel audio, creating more immersive sound for home theater systems by adding side or rear surround channels. It’s an upmixing/upconverting algorithm that uses proprietary processing to derive extra channels from less, featuring Music and Movie modes for different content types, and was common in receivers from the mid-2000s.
Dolby Pro Logic doesn’t have a flag in its spec to tell an AVR to enable this type of decoding. Some AVR’s do have an auto mode for it where they use heuristics to switch when they detect certain characteristics in a stereo signal, but typically it is something that someone manually turns on and off. If you don’t want it, just switch your AVR’s sound mode to stereo.