Home Xbox function does not work properly.

Not all content is shareable, the only thing you can do is buy those that don't work for sharing.

I do know from my own experience that Bungie did change the way shared content works compared to the last release. My own home has four Xbox One systems and we all play Destiny 2. We have 2 Gold accounts and with that my son and daughter can play all of
the content in Shadowkeep with the exception of the season pass content.

The easiest solution was to buy just the season pass itself from the Eververse market while in game. If I recall correctly it cost 1000 silver in game. Essentially $10 US. It saved having to pay for another copy of the game.

That's standard for most games. When I game share it's rare for both accounts to get the bonuses but it does happen with some games.

It's game sharing, not bonus content sharing. It's pretty stabdard for most games. If developers had that happen then everyone would just share a bunch and they lose money.

iareBrenden has linked the article officially released by Bungie in regards to game sharing. The key portion of that article is:

• Players accessing Destiny content through the game sharing feature may not be able to access
  all DLC, add-ons, promotional items, or pre-order items for which the primary account owns the licenses.
• Bungie does not support any issues with loss of access to items on accounts that are using the game sharing features to access Destiny content.

Your brother would still be able to see these items in the My Games & Apps since "Xbox Home" is assuming it's the account owners console/content. It doesn't check for whether or not he'll actually have permission to use the content in game.

I've seen a few threads saying that it got it to work by uninstalling and reinstalling the game, but if you have a slow internet connection I wouldn't recommend trying it since the majority of post point to it being non-shareable.