I am pretty sure your device has Japanese installed, as the popup (browser extension?) displayed the font correctly.
Possibly the web browser doesn’t force the Japanese variant, letting the Chinese one takes priority. Forcing can be done in the website, in HTML (in Discourse too), with <span lang="ja">. Inspect Element may elucidate the cause.
Desktop web browsers can be forced to prioritize Japanese, but more limitations on mobile.
A Japanese learning website for non-natives shouldn’t require the users to set the browser or the device to Japanese.