@James – THANK YOU! I wrestled with this issue for hours, until I finally found your post.
I used to have an ACF select2 event that worked prior to ACF 5.4+:
$("#acf-field_1234567890").on("change", function() { }
In your answer you mentioned that the selector is actually #acf-field_1234567890-input
. That solved it for me. I guess at some point ACF changed the select2 wrapper or version? At any rate, thank you! Your code above solved it for me.