Hey,
thank you for the report. This issue has been fixed around a year ago: b6a4d4a85148dfc0561981e7e65fcf2224ddb2a2 (reported here: https://gitlab.com/embeddable-common-lisp/ecl/issues/385). This change is not part of the release yet (fix was applied in 2017-05-30 while 16.1.3 was released in 2016-12-19).
If you are interested what we want to solve before we release 16.2.0 you may check out the milestone in here: https://gitlab.com/embeddable-common-lisp/ecl/issues?milestone_title=ECL+16....
Best regards, Daniel
(defstruct (foo (:type vector) (:predicate nil)) qux bar)