When editing text in the Finder comments field the suggestions for the spelling corrections are grayed out and cannot be selected, despite the fact that the field is editable.
See this screen shot:
Finder Comments are plain text.
Spelling suggestions in the contextual menu do work with rich text content.
However, the suggestions work with plain text in TextEdit, so perhaps @cgrunenberg has an idea on this.
The next release will fix this.
Fantastico! 