I noticed this was still occurring and didn’t see a BR. I have the latest version 8.5.3 and as well Reason 8 all updated.
1- create project with Reason activated and several channels, say 1-8.
2. On ‘Export Audio Mixdown’ select ‘Channel Batch Export’ and select your channel 3-8.
3. I uncheck the ‘Mono Downmix’ and nothing else in that list. Settings output are .Wav 44khz (same as my project), everything else is what I want here.
4. After Export the files are all created as stereo .wav’ even though each channel is a mono one.
The only way to get what I want is to use ‘Mono Downmix’, which processes during the Export making the export quite a bit longer in time to complete.
I’ve found that using ‘Mono Downmix’ does possibly effect the quality of the audio and seems to be in error of channel handling, here.