Please report bugs to Oracle using your MOS account.
That bug has already been reported. Workaround is to make sure the cursor is where it needs to be to enter the other characters/digits.
Just for reference... SQL Developer 4.2.0 - Typing letters in table data grid
Actually, this seems to be fixed in the main code line already, so please use the workarounds until whenever the next release gets posted on OTN.