Xbox game pass no longer shared across multiple xboxes

No luck. I performed the hard reset by pressing the power button for 10 seconds. The power went out completely. Then removed the power cable for 10 minutes.

Then, booted up the console -> logged into my girlfriend's account -> Games -> Full library -> Xbox Game Pass (shows as available offers) -> Selected a random game and instead of install i had the prompt to join game pass.

I've also tried removing my account as home console and setting it back again. No luck.

Try performing a hard reset by pressing the power button on the console for 10 seconds or until the power shuts off completely. Then unplug the power cord from the back of the console for 5 minutes.

Sounds like you are on about game sharing.

Have a read here,this may help
support.xbox.com/.../playing-across-multiple-consoles

If you run into any issues you can ask anywhere as that feature is not preview related.

Correct. You would set one of their consoles as your Home and then you can sign into a different console and still access your content. If you've set theirs as your Home and then it didn't seem to work, try going a hard reset (hold down the ppwer button
on the console until it turns off, unplug it, plug it back in, then turn it back on)

Yes and no. You can share your subscription via the Home setting, but you can only have 1 console set as your Home at a time. So they could access it if they use that console. In order to use all 3 consoles at once, you would need at least 2 subscriptions.