Thanks Syam,

I emailed you a copy of the plugin. And I got a reply from Elliot the developer of ACF;

Thanks for the bug report and sorry for the issue.
I have this on my to-do and will hope to find a way to customize the menu items without modifying the Walker class.
Unfortunately, I’m not sure if there are any filters or actions that I can use, but I hope there are.

So it sounds like he’ll look into moving away from the Walker in the future. Might be a good call to connect with him directly;
[email protected]

Appreciate you looking into the issue.
All the best
Cheers