Wordpress GUI and wp-cli are saying we have the latest version, but we don't

GUI and wp-cli are saying that we have the latest version, but we don't.

chanahelen@webarch5:~/sites/default/wp-content/themes$ wp core version
5.5.3
chanahelen@webarch5:~/sites/default/wp-content/themes$ wp core check-update
Success: WordPress is at the latest version.

5.5.3 is not the latest, the latest is currently 5.8.3.

Advice I've seen so far is to check the wp-cli version - this is up to date (2.5.0), and to disable any caching plugins (done).

I wondered if it might be held back by the PHP version; however, we are running php 7.4 which I believe is enough for the latest WP.