Microsoft To Do shared list – notifications arrive but the list on my account doesn’t update

I had the same issue..   I'm not sure why this occurs - perhaps the below may help MSoft to figure it out, it could be the link expires, or the device needs logging back into a microsoft account..  But :

3 phones, 3 people..  all using the same shared list.

My phone kept the most up-to date, but changes I made do not appear on the other two phones..   Notifications come in on those phones, but the updates to existing items do not show (new items do appear!).

The way I corrected this.. ( not sure if its the right way, but it worked! ) - On the two phones that needed a "complete refresh" of the complete list..   leave the shared list.   Using the phone that is working and has the latest data, re-create and re-distribute the link needed to re-join the list..   This was then clicked on the other two phones that previously left the shared list, they re-joined, re-loaded all the data the list and the data is now the same on all three devices and all three are updating correctly.

Not the quickest fix - but it seems to work and get the shared lists re-synced.