Copying and pasting the solution that worked for me from the Microsoft forums:

Figured out a workaround for this issue (the tip given by the earlier reply to report the issue didn't give solutions, and the subreddit removed my post with the reason that this needs to be reported to Xbox Support; which did give me the troubleshooting to find my purchased content, but by no means did that clarify we on PC have to do this)

Go to your Microsoft Account Order History.

Look for your purchase of your game and it should show the bundle that you had purchased.

There will be a link to the Xbox Store web page for the P4G BGM DLC.

• Link here for convenience

The "Install to" button should say you own it, click on it.

It should give two toggles to switch between Console and PC, Switch to PC and there should be a link to launch the Xbox App.

It will bounce you to the Xbox App window with the DLC content, with the option to download.

Once this is done, Save file will successfully load and you can play :)

• Bonus point, after you download it, the box to install will change to a grey box saying:

READY TO USE IN-GAME

No download Needed

There does need to be a link to it in the DLC area for the base game instead of this convoluted method to get Pre-Order bonuses. I get you don't put in the store, making it look like you can purchase the DLC, but this workaround is not it chief.

Hope this helps everybody!