I guess it is just a matter of waiting then.
You could try to make your own TextEdit/LineEdit node if you want, but it might be more trouble than it is worth. Does the issue occur with Godot 3.0, or just Godot 3.1? If the issue does not occur in Godot 3.0, you might be able to use Godot 3.0 for development while you wait for the bug to be fixed.
Outside of that, I'm not really sure. You could probably write GDScript code that deletes the extra characters when a new character is added as a workaround.