Hi @claire
This is probably not a bug but a configuration issue.
When working with WPML the easiest workflow is to create your field group in the default language and then use the ‘Duplicate’ tool to create a duplicate field group in the new language. You can then edit the field’s label, settings but the field’s name must remain constant across all translations.
You can take a look at the following tutorial on this here: http://www.advancedcustomfields.com/resources/multilingual-custom-fields/
