This morning, I had ~4500 photos, all in Photos albums. All had been uploaded to iCloud in the last couple of days, and were represented correctly across multiple devices. There was no further iCloud activity taking place.
At some point between then and this evening Photos has, without any warning that it would or had, deleted a very large number of those albums. There are now ~2500 photos not in any album. All the photos seem still seem to be in the library, but albums are gone (on iCloud, and across multiple devices).
How is this possible?
Albums are affected randomly, there appears to be no pattern to it.
I haven't been editing photos or albums since this morning.
I suspect that it's some kind of database conflict in iCloud, that it resolved by deleting these items.
I did turn on an old iPad (iOS 9.3.5) this afternoon, that's connected to my iCloud account, and opened Photos. Conceivably that's somehow related, but it was already connected to the account.
I have a TimeMachine backup, but it's hard to guess how an old Photos library database is going to interact with iCloud.
I completely wiped the iPad I mentioned since writing the above - a complete reset - and allowed it to reconnect to the account.
I watched the photos and albums appearing as it downloaded the library. After a while, some of the missing albums appeared on the iPad, in their expected places.
The same albums were not showing up on icloud.com, the source that it was downloading them from.
And then a few minutes later on the iPad, again before my eyes, I watched them disappear again, popping out of existence.
I still see some of the "missing" albums showing up on the iPad, downloaded in the last minutes, though they're not visible via icloud.com - oh, but they are now: literally while typing this sentence, they deigned to appear.
Will they remain? Will they disappear again? Who knows?
I will leave this car-crash of software overnight and see what's left in the wreckage in the morning.