I have been able to duplicate this behaviour and I'll be darned if I can explain it. Yes the menu is enabled, yes the Select all (Ctrl+A) is enabled but Ctrl+A does NOT select the text in the EditBox, although it will select the text in TextBox in the same grid.

Maybe you can convince your user to use Shift-End to select text? That appears to work in both although it only selects from cursor to end of field.