Okidoki. So excuse the short notes and lack of formatting here, but my other half's laptop died so, of course, I'm on my phone while she has the laptop for editing!
There's definitely something up with the interface of this flanged freehub I've got. I've played around with the two wheels easily accessible, a 2016 built 240s 142x12 CL and a 2019 built 240s 148x12 6-bolt, both of which should be fine according to compatibility charts.
Apologies
@Nambra but my calipers are a bit too simple to get all the internal and differing measurements you wanted (they're the basic Kinchrome digital from Bunnings), but they did highlight that it's definitely the MS freehub.
I've copied all my measurements below, but to sum it up, both Hub axle spacers were fine, the hub widths using XD and HG drivers were fine, while the MS driver pushed the widths out about 2.5mm on each Hub. I think this can be attributed to the freehub lip not extending past the flange, compared to the XD driver - a comparison pic below with the flat base of each driver lined up (again excuse the dodgy late night shed photo!).
Non Boost
Hub with XD - 141.9mm
Hub with MS 144.6
Spacer 15.4
Boost
Hub with HG 148.05
Hub with XD 147.9
Hub with MS 150.6
Spacer 15.41
I didn't even need to install a cassette, with the MS driver on each, when installed into the frame and the axle done up, it bound immediately. What's happening is that the bottom of the freehub is binding on the hub shell as the axle is tightened, as it's not recessing into the hub seal correctly.
Spacing out the freehub might work, but that's then going to leave the internals open to the elements, definitely not a path I want to take.
I'm totally stumped and really the best idea I can think of is to either find the flangeless freehub or get it machined. I can't believe there are two versions with essentially the same part number. I'll probably have a chat to Apollo or Dirt Works tomorrow to see if they have any thoughts.
I haven't seen anything like this in quite a while! I've got an entire stable of DT hubs (6 of 9 bikes with either 240 or 350 hubs) and the idea was that they were all interchangeable with components. I guess that still stands as it seems to be the freehub that's out of spec, but I'd like to figure it out so that in future I can know that all my drivers are usable on each bike.
Any insight you all can provide I'm open to!