Seeing many posts related to this subject recently. We found a solution and it's working. hope this can work for you too.
I was working on the Electrical Revit model in Revit 2021.1.3. we provided all electrical equipment tags value as a shared parameter and those were working well. and suddenly one day, all tags changed to "?". but the parameter value still exists there. When we tried to use the same tag one more time, it is perfectly coming normal. but retagging all is a hell of rework. so I researched some alternate methods.
Then I found that the below step is perfectly working and all the tags back to normal as before once this is done.
Solution
Go to manage tab - open Project Parameter - Select the Parameter (the parameter that becomes "?" on your model)
Click on "Modify" - Select "Export" - click ok

then Select the "?" family - Edit Family - inside the family edit, go to Manage tab - click on Shared parameter -
here you can find the parameter you exported earlier under the "Exported Parameters" Group.

select the parameter you want - click ok.
Then -go to Edit Label - Click on "Add Parameter - "Select" - select the Parameter from the shared Parameter - click ok
(*Some cases maybe you need to remove the existing Parameter added with Label and add again)
once you edit the label as you required (using the shared parameter), Load this to the project.
This will make all your "?" tags Back to normal.
Hope this can solve the issue