So, excusing my ignorance as a fairly recent Linux convert, what does this mean for my dual boot system?
I haven’t booted windows for weeks and am pretty sure there have been no updates since it was freshly reinstalled (maybe 6 months ago) as a dual boot with Debian.
Is this only a problem if I allow Windows to update?
Are Microsoft likely to fix the issue in a subsequent release?
Yes, you don't have to worry as long as you don't boot up windows and let it install the update.
This is not the first time they break dual boots by touching the partitions, but this is the first time they deliberately break it (that I know of).
I always had windows on its own drive because of that. If you don't use windows a lot then I would suggest to do the same. You have to change to windows through bios but it isn't that much more work.