I couldn’t tell you if this has anything to do with the error or not, but the code you mention is run and you can see the result in part of that JSON string
... "relationship":{"max":"Maximum values reached ( {max} values )","tmpl_li":"\n\t\t\t\t\t\t\t<li>\n\t\t\t\t\t\t\t\t<a href=\"#\" data-post_id=\"<%= post_id %>\"><%= title %><span class=\"acf-button-remove\"><\/span><\/a>\n\t\t\t\t\t\t\t\t<input type=\"hidden\" name=\"<%= name %>[]\" value=\"<%= post_id %>\" \/>\n\t\t\t\t\t\t\t<\/li>\n\t\t\t\t\t\t\t"} ...
I would be looking for other JS errors that happen prior to the error in ACF.
