This has also been reported in Issue23598 including a patch to add DEL as suggested.  I have little experience with curses so I don't have any insight as to pros and cons of using erasechar() rather than the hardcoded BS and DEL chars.  A test would be nice if someone has a creative approach for creating one.  I see that a patch for Issue13051 contains a suggested test for another problem with curses.textpad; perhaps it could be adapted.