Seems to be a problem going back to emacs-26.3

# all good
src/emacs -Q --batch -l ./usurped.el

# buffer-local causes let-bound variable to freak out
src/emacs -Q --batch --eval "(setq confuse-thread t)" -l ./usurped.el

# buffer-local causes basic setq to freak out
src/emacs -Q --batch --eval "(setq confuse-main t)" -l ./usurped.el