I have a onePlus 3 (which anecdotally has been a lovely handset, just extremely fragile. And onePlus and their repair company have been entirely useless at communication...) I also have the EngineerMode installed and it's also using data; "61.34mb since 1 Aug". It's worth noting that the data usage (752kb since Nov 1st) says it also includes other apps, I've listed them below for reference as I've not seen anyone els…
Looking at the data usage for several of the connected apps (my list is identical to yours as far as I can tell), it looks like the only data they send is as a subset of engineer mode (their individual data sent isn't shown, only the engineer mode total).
There's definitely some concerning names there. Double checking my recent screenshots, it seems at least that it couldn't be sending full images with as much data as it's used. It's likely that it's not sending data from all of these, but just accessing them at some point. The previous leak on here revealed that OnePlus could track when you opened and closed apps. Based on this, it could potentially track your location, when you take screenshots, when you make phone calls, and a host of other information.