Following on from the previous entry, where I outlined a weird problem I'd started having with syncing Obsidian via iCloud, I finally decided to sit down and try and work out the exact flow of the problem. Today, for example, I'd created an entry in two different vaults on my phone while on the bus into work, and when I got to my desk the vault I use on my work machine had updated.
However, when I got home this evening, the vault for my personal stuff hadn't updated on my home Mac Mini. I tried a few edits, in both vaults, on the iPhone, and nothing came through to the Mac.
So... before I started really diving into things I decided to "turn it off and on again" -- the iPhone that is -- and when it came back I ran up Obsidian, which told me it wasn't allowed to access my iCloud drive!
I took a moment to go into the settings to try and figure it out, didn't find what I wanted right away, then got to thinking that perhaps some of the phone's services were still spinning up, so I ran Obsidian up again (after killing it).
Sure enough, this time, it saw my vaults. With both vaults open on my Mac I made edits to open entries and the edits started to flow.
So, yup, looks like it was a simple case of "turn it off and on again".