Original report on forums
Summary:
When working on an Arches installation with multiple languages and concepts with multiple prefLabel values for each language, the different labels (one for each language) for the concept have different IDs and when you save a record with a concept, it uses the ID of the label in the current language rather than a generic ID for the concept that is independent of language. This means that two records with the same concept chosen but in different languages will have different values in the data. This causes unexpected results including searching for records by matching concept in Advanced Search only returning the records that had the concept saved in the current language.
More information can be found in the linked forum post.