It seems that the problem is linked to a custom filter I added to sanitize_title that doesn’t allow underscores and convert them to dashes.
The problem is that ACF checks for field key to be like “field_***” (function function acf_is_field_key() in api/api-field.php) ; so if the field key is “field-***”, it doesn’t work.
Another solution is to activate local JSON so your field key can be what you want. Just create a folder name acf-json on your theme folder and re-save your field group.