Have a look at the Extend() function.
I've tried putting Extend() around all possibilities and still no luck.
It's not a matter of luck; you need to use Extend around every non-repeating field referenced in the calculation. I don't see the full context of your file, but it would probably be something like =
cAssignmentGantt = "Received" ; Extend ( Control::gRed ) ;
cAssignmentGantt = "Deadline" ; Extend ( Control::gRed ) ;
That did the trick. Thank you, thank you, thank you!