I found this issue: https://github.com/nathansobo/treetop/issues/25, and it appears to have answered my question.
My grammar did not contain a top level rule that would allow an opening or closing tag, therefore the second possibility was not even considered:
grammar BBCode
rule document
(open_tag / close_tag)
end
rule open_tag
("[" tag_name "]")
end
rule tag_name
[a-zA-Z\*]+
end
rule close_tag
("[/" tag_name "]")
end
end