In my XD document I have a date on each page that I have created a symbol for so it can be easily amended before sending to the client.
However, when I go to edit the date text, it is now not changing across the board (it is not ungrouped and retains the green symbol box). It was working previously...
This is most likely caused by the recent "Symbol override" feature. Text and bitmap changes can now be edited per symbol instance.
In your case, you should be able to right-click the date symbol instance you'd like to propagate and select "Update All Symbols." That should update all other instances of that date symbol with the text of the instance you selected.