I would need a lot more information to figure this out, however, a starting place is that group fields behave almost exactly like repeater fields and must be saved in a similar way. So if you’re only getting a single field that is in the group field for your form and not the entire group then this will be a problem. If you’re doing this then ACF is probably seeing the field as just a top level field and not as a sub field of the correct group.
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