Changing field configuration results in catastrophic data loss

I have a situation where, within a field collection, if I change the field configuration in a way that should be non-destructive - for example change the label or default value for a field, or if I add another allowed value key/label pair to a list field, or change the label for an existing list field key - then all entities using that field collection will display those fields with undefined values on next edit.

All rows for the field collection in the field_data_{fieldname} table are removed (!)

This is not the case for the same field used directly in a node, where I can do any of the above without affecting existing field data in the node. Those rows in the field_data_{fieldname} table are unaffected.

Has anyone else seen anything like this?