It would have to be happening in a filter somewhere on the site. ACF by default does not add the ability to disable fields and it must be done in a filter.
I would start with a search of the code base for acf/load_field or acf/prepare_field filters. It’s possible that a filter on some other field is affecting this select field as well.
Another possibility is that the acf/load_field filter affected the field while a field group was being edited and ACF save it in the field settings for the field. acf/load_field can affect how fields are saved, for example if an acf/load_field filter is used to add choices dynamically to a select field, these added choices will appear when editing a field. Whereas an acf/prepare_field filter will not have this result. Generally it is better to use an acf/prepare_field filter rather than an acf/load_field filter for this reason. But the acf/prepare_field filter is newer.
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