Long container titles can exceed the width of the fieldset legend, wrapping outside of the fieldset itself.

The preferred truncation is probably in the center of the text, which is why this may not be a simple theming issue. E.g.:
"SERIES II: J. Adams, S. Adams, J. Bartlett, C. Braxton, C. Carroll, S. Chase, A. Clark, G. Clymer, et. al. to George III, A Declaration. 4 July 1776"
might become
"SERIES II: J. Adams, S. Adams, J. Bartlett, C. Braxton, ..., G. Clymer, et. al. to George III, A Declaration. 4 July 1776"