Could you elaborate please. I can't figure out what in the input-mask makes it only sometimes behave this way.
This is what I find in the help file:
When you type data in a field for which you've defined an input mask, the data is always entered in Overtype mode. If you use the BACKSPACE key to delete a character, the character is replaced by a blank space.
But I have some input masks that behave this way, and some that don't.