Consequently, if you bind a DOM field with an empty string to your proxy object, it will not get passed at all, as opposed to being passed as an empty string. While there is an argument to be made that it should work that way, ACF does not work that way.
This is resolved in this PR (wasn't sure whether to submit it to the extension or the Lucee core):
https://github.com/lucee/extension-ajax/pull/3 – we just comment out the two 'if' lines. Presumably they were put there for a reason, so the comments were left in until this can be reviewed properly.