Recently I was playing a game on my Steamdeck through family share for a week. I only used suspend and wake feature while in the game, and never closed the game process. I later found out the owner refunded the shortly after purchase, but Steam did not kick me out of the game. I finished the game, and I was online the whole time.

On a separate occasion, I played a game for 10 hours through family share, and then decided to get a copy of my own. Wanted to return it after 15 min, but Steam won't let me because it is considering prior 10 hours play time makes me ineligible for 2 hour refund window.

Unfortunately the Steam client is simply abysmal offline. Even basic functionality such as caching game title thumbnails and viewing screenshots appears to be nerfed on purpose. You can't view your own achivements. Some games simply won't launch on the Deck offline. Now I will always try to buy a game on GOG first if it's available to avoid this sort of DRM.