Manually upgrading using ha os update --version 5.10 worked just fine, only the update wasn’t showing up in the dashboard - as above JSON is apparently used for checking for new versions.
New version of Home Assistant Core was released a few hours ago and is already reflected in mentioned JSON. There seems to be an issue with Home Assistant OS only?
I was wondering too. Basically - as I learned during the last two weeks full of suffering and issues since running on HASS OS 5.8 - I´d meanwhile say “plan updates wisely” especially the OS usually doesn´t offer really important new features, stability > features.