90% of the time when this happens it is due to a conflict with something. It could be a another plugin in something in your theme. More often then not it has been a pre_get_posts filter that is interfering with the acf queries.
You need to try to narrow down what it is by deactivating plugins and maybe switching themes.
If you do a search in this forum for “image field returns id instead of array” the topics that come up may help you.
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