Auto completion for partial arguments

I'm trying to get bash auto completion working for my string arguments based on hints that I'm correctly defining in my app.

"myapp --completion-bash" is correctly returning a list of custom hints I've specified for the argument.

But "myapp a --completion-bash" does not return a subset of the previous list based on which ones match the prefix 'a'. This means that bash won't use tab to auto complete the argument.

Is there a way to achieve the desired behaviour?