The UIA entry for figcaption seems to be missing the following:

Other properties: The LabeledBy property for the parent figure element points to the UIA element for the figcaption element

See "caption" and "legend" for example text.
See also that MSAA/IA2 and ATK do set up "...RELATION_LABEL_FOR with parent figure element" for figcaption.

@melanierichards Can you confirm that this is true?