I’m experiencing a similar issue. This is with the ACF plugin and the gallery add-on both fully updated.
@Jonathan, it definitely does not work the same way as the built in gallery function, but consistency between two different ways of adding images to the gallery field is a reasonable expectation. Either the images should never be attached to the post regardless of how they’re added (not ideal, IMO), or they should always be attached (ideal, IMO).
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