As stated in the original post, Silent Mode (in theory it is DND too) is on. I checked the Performance tab as per the link. There's no switch like the link describes, but all four boxes available are/were checked.
I wasn't disturbed today (over three hours time). My Win10 did restart my computer for updates, so maybe (or maybe not) that had something to do with it working correctly.
I'll keep and eye on it and report back over the next week.
Cheers!