If you are updating the value on the other end where a value was not previously stored by ACF and you are using the field name when calling update_value() then this will cause the problem you are seeing. When you use update_field() on a field that has never been saved before then you must use the field key instead of the field name. This in explained in the documentation https://www.advancedcustomfields.com/resources/update_field/
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