🚀 go-pugleaf

I'm experiencing a rather disconcerting problem with a dialog box I'm
trying to manage.  The configuration is: 1040 ST, color monitor, Mark Williams
C v2.1.7, K-Resource.  The problem is that the dialog contains some 3-digit
numeric editable fields--I sprintf() numbers into char[4] variables, then set
the appropriate te_ptext pointers to the unsubscripted names: at first it seems
to work fine.  The numbers in the editable fields when the form is first 
displayed are the correct initial values; if I select one with the mouse and
hit <ESC>, the field is cleared.  However, if I select one with the mouse and
backspace over it, the rightmost digit is not erased--the other two digits are!
Furthermore, once I have the cursor at the beginning of the field, when I type
numbers into it, the first number typed just moves the cursor; the second 
number typed echoes the first, the third echoes the second.  After sscanf()-
ing the strings after return from the form, then redisplaying the form, the
values shown are appropriate to what was actually typed in, not to the odd
stuff that was shown in the form.  Can anyone tell me what's happening, and
what to do about it?  I've done similar things before, but with longer strings.
For reference, the validation characters for the field are XXX