Well that’s good. But since the new code you posted does work, I’d start checking things like viewing your browsers source code for the page and verify that your custom script is indeed loading after ACF’s acf-input.js. Perhaps there is a problem there, even though your wp_register_script is setup properly.
Also a theme can cause odd behaviors on plugins depending on what it’s trying to do. I’m just in the middle of developing an add-on plugin to ACF and when I popped that code in, it worked great.
I’m sorry I can’t be of more help at this point, but it seems like you DO have the file enqueued properly, just a matter of why the acf object isn’t able to be accessed. Which is odd, because ACF attaches that object onto window, so it should be available anywhere.
