Probably not the reason, but worth to do a quick check:
Sometimes the update cache interferes, so if something's odd with updates - some module (or core) claims to have updated, but then...
The issue with checking for updates in the BackdropCMS dashboard makes website development and maintenance more difficult. I manage more than ten websites hosted by two different providers, one...
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This issue (with PR) has been raised by a user using CiviHosting:
https://github.com/backdrop/backdrop-issues/issues/6759