I have this issue with parameter descriptions.
The tooltips are visible in modify mode, e.g.:
but If I go in edit mode, the description field is empty:
If I connect to the .pbix file in Tabular Editor, the Description attribute of the Parameter object shows the proper text:
If I edit it, the change is saved in the 'local' copy of the datamodel in Tabular Editor, but then PBI can't save the changes in the .pbix file.
If I export the .pbix as a .pbit, then the descriptions completely disappear:
I tried the latest version of Power BI Desktop and can see the description. You may leave a comment on GitHub.