When editing a work, the code is meant to empty the value field when you change the attribute type (see root/static/scripts/work.js). That does not work though (it always compares the ID after the change), so the ID is always kept, even when it doesn't make any sense - for example, if you change from an attribute with a fixed-value select to a free text attribute, the ID from the selected value will now be the free text value for the attribute.