Why does my sourced script only work once?

Question

I'm currently trying to refactor a software of mine. I wrote this script:

shopt -ps cdable_vars
. ~/.config/butler/butler_aliases

while getopts 'a:' OPTION; do
  case "$OPTION" in 
    a) 
      echo "Adding stuff. $OPTARG"
      echo "$OPTARG=$PWD" >> ~/.config/butler/butler_aliases
      return;;
    ?) 
      echo "Usage: $(basename $0) [-a]" 
      return;;
  esac
done

It's meant to be sourced in the current shell to be able to resource the alias file immediately. It all works great for the first time:

u@h:~/.local/bin$ . butler -a test13
Adding stuff. test13
u@h:~/.local/bin$ . butler -a test14
u@h:~/.local/bin$

When running after the first invocation, it does not do anything, which I don't quite get. Can anybody explain why it behaves like this?