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If it is a sub field then you cannot use update_field() to update it. This function is only for top level fields. To update sub field you must use update_sub_field() or use some other way to update the entire field and all sub fields at the same time, as I outlined it a this solutions that I referred you to in a previous reply https://support.advancedcustomfields.com/forums/topic/field-value-update-using-update_field-doesnt-show-on-backend/#post-162342
It is not that confusing, if it is a sub field then you are using the wrong function/method to update the value.
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