Anonymous users submitting blank values on profiles with Select2 lose existing data on dedupe match.
This is identical to CRM-10128, but doesn't handle the case where an element's value is an array. Technically this bug existed when CRM-10128 was around, but you'd need to be using the legacy multi-select field, which was pretty rare. With Select2, this is more feasible as a scenario.
To upload designs, you'll need to enable LFS and have admin enable hashed storage. More information