WordPress does not auto update its core

I am taking care of a site running 4.9.8 which unexpectedly did not auto-update to 4.9.9. No one disabled this feature in wp-config.php or anywhere else. The files are owned by the server process.
I’m asked to manually update to WP 5.0.2 in the dashboard but I’d like to stay on 4.9 branch for the nearest weeks.
How can I find the reason for lack of automatic updates?