You're already running 18 hours of light, if it's flowering it must have ruderalis genes somewhere along the way. Dunno if a reveg will even be possible. It could be a genetic anomaly and it just has very prominent preflowers that doesn't progress further until 12/12 but I doubt it. If it is an unintended autoflower I would just continue course with the plans on the other plants. The auto will do its thing and finish before the rest anyway.