This issue could be due to a conflict in the Select2 versions. You should look into what version of Select2 the other plugin is using.
ACF has a setting that you can use that might let you force it to use the same version. https://www.advancedcustomfields.com/resources/acf-settings/
Alternately, if there are no ACF fields on the page in question you might be able to unqueue the Select2 script enqueued by ACF so that the other plugin can properly enqueue its version. I’m not 100% sure of the details for doing this. Here are a couple of previous support topics that might help you https://support.advancedcustomfields.com/forums/topic/select2-conflict-with-awesome-support/ and https://support.advancedcustomfields.com/forums/topic/select2-version/
