' fields can have TML in their names. The form subsystem handles that by (I guess) removing non alpha characters from the field name when storing that name in topic metadata. When presenting a form, the field name's content is expanded, but isn't when editing. This is important when writing internationalised applications. See, for example, TWiki:Codev/TranslationSupportProofOfConcept
The idea is to expand the content of the field name for when it is shown to the user in edit screen. This won't affect how forms are stored, or anything else related to forms. This does not affect either field names that are topic names pointing to the topics that define that field.
Reopening. When select values have variables, the value shown could be the expanded variables,
and the stored could be the unexpanded.
Deep stuff. See TWiki:Codev/StoringFormValues
This is an enhancement, AFAICT - at least, there is no obvious bug described above.