I'm marking this one for further review at some point by our QA Analyst, I gave it a try but couldn't replicate either on this level or in a clean sandbox map.
Worth noting that the Sappers on this level are slightly different from the default sapper so maybe something to it. I also witnessed it on a Stream last night.
Cian will review this at some point though it may not be until the new year. He'll be able to look into the save file as well in a bit more depth than I can.
Confirmed this behaviour in Hollow with a sapper, incredibly good find answers a few questions as well such as capture-able sappers.
Thanks a lot for this one, I don't know how you keep finding this niggly ones that have gone undetected before :P
During mission 3 of HoG your workers can capture enemy sappers. savegame