"Bring the last scabbard of Akrash to Ghorza" is shown on the map at Fort Sungard, which the wiki describes as Forsworn occupied.

However, it's now held by Stormcloaks, who I don't want to fight.

A workaround is to obtain the other copy of the book from Gallows Rock.

I've reported this on the Unofficial Buglist, but would welcome any insight folk may have on why this happened.

It obviously isn't the well-known bug that quests ignore events in the Civil War. I did kill the Forsworn "king in rags" Madanach in Cidna Mine prison - perhaps some Forsworn sites change hands as a result?

If you are friendly or neural with the stormcloaks they will not attack you so you can probably just walk in and get the book.
Know they can take over bandit forts I have cleared pretty fast, did not know they could take forsworn forts.

I'm neutral, but they behave like it's a restricted area. Outside, you get a warning first, but if you sneak into the fort, you just get attacked.

I was afraid this would make Stormcloaks hostile elsewhere, so I reloaded and went for the workaround.

Some additional facts:

I had not discovered Fort Sungard before this happened.

Earlier, Markarth was transferred to the Stormcloaks at the MQ peace conference.

Perhaps Fort Sungard is incorrectly reassigned to the Stormcloaks at that time?

Had you cleared the fort once before? In my experience, clearing some forts yourself can cause whichever faction controls that region to use it for their own. I was neutral when I cleared a fort between Whiterun and Markarth (no peace treaty yet, had just started) and the next time I went by Imperial soldiers were inhabiting it.

No - I definitely had not even discovered Fort Sungard before this happened.