Hi,
I'm not sure why it has started not showing Greek characters in your situation. On my machine, it works fine. You can check the following:
1) On the text label, right-click, and select "Properties" flyout to show Object Properties dialog.
2) Add the following string at the end of your string in the text box:
\f:Arial Greek(\(a225)\(a249))
3) After press OK, do you see now Alpha and Omega in your text label?
Also, using "Symbol" font may be a work-around for you.
--Hideo Fujii
OriginLab