Are you sure that you always saved your document after you shared? This is necessary to keep the link associated to the file.
If you are sure that you always saved, could you explain a little bit more about the situation? In this case it would be a bug, as the link should never get lost.
Same thing happens to me. I'm constantly having to make new links and resend links to clients, which is goofy. My account just gets filled with adobe xd links now. Even something as simple as swapping out one image, still need to create a new link. http://i.imgur.com/ViJ5UMe.png
So for you, it looks like you're on the Windows version of the product. Updating a link will be coming soon! Upvote here: [Windows] Update prototype – Adobe XD Feedback : Feature Requests & Bugs
I've been using XD for 6 months and have been having the same problem and I'm on Mac. Definitely saved my document after sharing. There is no ability to update the link. Have you guys addressed this?
If you want to update a link after making changes, click the "Share Online" icon, located at the top right of your window.
This is where a lot of people are overlooking how to update your link. Instead of deleting your previous link and creating a new one, just click the "Share Again" option. From there you can choose either "Update Link" or "New Link", Choose "Update Link".
If you see "Updating shared link", you're good to go!
I hope this helps!