That is currently expected.
Ubuntu 26.04 LTS, which includes all flavors like Lubuntu too, is only released and stable for new installs, as only the ISO release has occurred.
I’ll refer you to an answer I wrote somewhat recently.
(my reply is 2nd post on that thread but the question related to a Ubuntu 25.10 upgrade to Ubuntu 26.04.
If you check the meta-release file you’ll see a “Supported: 0” against resolute or 26.04, as upgrade is not currently supported.
Ian’s response too (@g-schick posted) is also good advice; ie. reference to Best practice for a successful release-upgrade to 26.04
Ubuntu 25.10 to 26.04 will upgrade soon, be it days or a week or two, but we’re not at that stage yet.