Mac terminal Vim will only use backspace when at the end of a line
Most likely, the "problem" you're seeing is that you can't delete anything that was not typed during your current insert mode session. This is due to the default setting for the 'backspace' option. Adding set backspace=indent,eol,start to your ~/.vimrc is the behavior that you probably want.
This is the only explicit backspace mapping I have in my config. I do not know if it will help for your problem, but it might be worth a try?
" allow backspacing over everything in insert mode
set backspace=indent,eol,start