Given the "upd" tag, this could be a announced on social media (Twitter/X, Pixiv, or YouTube). Independent anime creators often release "trailers" or "production updates" under hashtags like #himawariyorusaku. A search in Japanese (向日葵は夜に咲く) reveals scattered fan art and short animations, but no complete OVA as of late 2025.
Himawari Wa Yoru Ni Saku: A flower blooms in a time of crisis himawari wa yoru ni saku ova sunflower ha yoru upd