This one killed about 6 hours for me last night:
I started a fresh project on my laptop, using a UR22 as interface for monitoring (just for the record), with all of my working mono tracks (two sources) routed first to individual mono Pre-DX groups, which were in turn routed to a single mono DX group.
Got back home and transferred the project to my home computer (Fireface 800 as interface, fwiw). Did all the editing and set it up for batch marker export, with the output as the mono DX group.
Started the export, walked away, went to sleep. Woke up to find all of my exported masters were stereo files. Checked the export dialog for any departure from my usual setup; there was none. Tried doing some individual exports: all came out stereo. I thought about using the “mono downmix” option, but had no confidence in the ultimate levels, and didn’t have time to spare experimenting.
So… I saved the various FX chains for the group channels, made track archives of the audio to be exported and markers, and imported them all into a fresh project. Just tested it out. Everything exports the way it ought to.
I have no clue as to what could have gone wrong. Settings are identical between the two projects; all groups are mono. All plugins are UAD2. Only difference I can discern is that one works; the other doesn’t.