Microsoft Sudoku has once again lost my progress on daily challenges. Please advise when this issue will be fixed and my progress will be restored.

Thank you this worked for me!

Having tried all other suggestions I tried this, and it DOES work but only to a limited extent.

Provided I open the XBox app first, and only then use it to open MS SuDoku, then my Daily Challenge acheivements are not only recorded but - so far - also retained permanently.
This despite me still getting the "Microsoft Sudoku must be connected to the internet to play Daily Challenges" warning, which - before this fix - meant that all acheivements were totally forgotten when I next went to open the app.

NB If I now go in to the app directly without using XBox, then I still get the error warning, and - as before this fix - no acheivements are ever retained by the app.

So thank you indeed for this work-around to enable my acheivements to be saved.

However I do find this a clumsy way to get around what appears to be a problem at the Microsoft end: if they can't be bothered to fix it, then very soon I just won't be bothered to play this game.

Same for me... tried this too, but no luck..
Still losing progress when closing the sudoku window

I tried this and no fix. As much as I am sad for all of us who are having this problem I feel better that it is not just me. My husband is a programmer and followed the fix instructions exactly, it didn't solve the problem and he has no other suggestions. Has anyone tried anything different?

You have to use File Explorer and you have to do a couple things first.

Click on the 3 dots then choose options like this:

Then from the View tab, enable the Show hidden files, folders, and drives setting:

The last thing is to substitute your actual user name before drilling down to the C:\Users*{your user name}*\AppData\Local\Packages\Microsoft.XboxIdentityProvider_cw5n1h2txyewy\AC\TokenBroker folder.