ACF doesn’t have it’s own cropping feature but uses WP Cores so it should be the same no matter if you crop the image directly or via media library.
You say you’ve used the “image crop plugin, for ACF”. Is this some third party plugin you’re talking about?
Also, please note that the “preview” of an image you see when adding an image to an image field can be whatever size you choose when creating the image field and does not reflect what is being returned when you use the field value in your theme. Perhaps that’s what you’re talking about?
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