AVR routs monomixed parts of DD2.0 content from TV apps to center even when not upmixing?

[XaVierDK]

The setup:
AVR: NAD T758 v3
TV: LG C3 OLED

After a longer period of running 2.1 and 4.1 for several reasons, I finally had the chance to reintroduce a center channel speaker to our system.

Set up things as usual, ensuring no upmixing was selected as default and so on.
Watching 5.1 content from the TV's built-in apps are played back perfectly with no issues. However, all 2.0 tracks from the TV when outputting bitstream result in any monomixed parts being muted, but only above the crossover point.

If I turn on Dolby Surround, the missing audio is output correctly through the center channel.
Stereo tracks from streaming on the receiver are fine, as are 2.0 PCM.
I have reset the HDMI ARC handshake and reset the TV.
Do you guys have any suggestions or possible causes? My next moves are a reset of the AVR and a new HDMI cable.

This TV and receiver have previously worked with a 5.1.4 setup with no issues pertaining to stereo playback.


Thanks in advance

 

[XaVierDK]

Since posting I've gone through a factory reset of the receiver and replaced the HDMI cable to ensure no hardware defects. These did not resolve the issue.

As it turns out, when setting up the receiver for a 3.1 system is where the fault occurs. If I enable the surround channels, the 2.0 Dolby Digital stream is decoded without issue and plays properly through the front L/R speakers.
I can't figure out why though it seems a firmware error, and seeing as the 3.1 setup was only an intermediate step, the solution seems to be to just get hold of a pair of surrounds ASAP and live with the Dolby upmixing for 2.0 DD content in the meantime.
I do recall us at some point running a 3.1 system without issue during a remodeling, so it might be something introduced in a firmware update sometimes in the time since then.

Best regards