Whenever there is a problem with an ACF image field returning just the ID instead of the url or array if it is set for this, 99% of the time it is caused by a pre_get_posts filter that is messing with the queries done by ACF to get the values for the image.
Your first step is to disable other plugins and possibly switching themes to one of the generic 20XX WP themes. This will narrow down where you need to look for the offending filter.
https://support.advancedcustomfields.com/forums/topic/image-field-returns-id-not-array-when-filtering/
https://support.advancedcustomfields.com/forums/topic/get_field-from-wp_term-return-id-on-archive-page/
https://support.advancedcustomfields.com/forums/topic/issues-with-pre_get_posts-and-get_field/
