A follow up on this:
My Mac died. I ended up buying a new one.
I installed PixInsight, and on the first start it found new updates (same as on the old machine), quit and let it install the updates.
Noticed that one of them was documentation.
After quitting and restarting when updating was finished, documentation is now present.
I suppose its possible that the old machine was getting glitchy towards the end of its life, although the eventual killer appears to be a dead graphics card.
Anyway - for anyone interested, I think this falls into the category of "not a bug".
