So, your mission is to show me where I screwed up. Where on the various store and official specification pages has MS hidden the fact that this game uses always-on DRM? The closest I've found is:

"Xbox Live and broadband internet required for initial download of game to console."

which make it sound like the exact opposite for consoles, ie only the digital download needs an internet connection. The WinStore page talks about online but at no point do I read anything which implies that not being connected to the internet means the app simply doesn't run.

So my router just caught fire (figuratively) tonight and we've been offline for a few hours and I thought I'd use the time waiting for the outage to end (this was before we realised it was the router, not an area outage [we're currently designated as a low service area due to a recent storm], which had knocked us offline) to play some Horizon 3. Can't play co-op or check the online leaderboard etc but my progress would all sync when I got back online because this is a standard thing with cloud saves (which Horizon definitely has because you can move XB1-PC and continue your save).

Only I ran the app and it instantly closed. Fraction of a second of the loading window and then gone. Tried it several times and rebooted, same thing. Forza 6: Apex loaded up just fine! As soon as we get back online, Horizon 3 loads up normally.

So did I miss when MS said this was an always-on DRM game? Is there any text on any of their store/official pages that makes it clear this game doesn't even try to work when your Windows PC can't detect an internet connection?

Yup. I just tried it and it's exactly like you said: the game exits instantly when I unplug my Ethernet, but boots right up when I plug it back in. If I were you, I would air out my grievances with Microsoft support. If they really didn't disclose it on the store page (not doubting you, just didn't look), you should be able to get a refund.

I just tested it on my Juan, and it definitely works fine without being connected. It seems super nuts to me that it is DRM for PC only, but based on that quote of yours and the fact that it works on the Xbox, I guess it is? I do have the disc though.

Best of luck getting it sorted out.

The game definitely requires an connection to start up but I've disconnected my internet after the initial load and have been able to play without issues. This is on the PC version.

Well it is microsoft, they made no attempt to hide the fact that this is what they want to do. To bad they hid it in this game and i guess hoped nobody would notice?

Ok, good to know. So not always-on DRM but on-load online DRM. At least it won't kick you out of the game when you're in the middle of a session and your connection blips down.