TL;DR We’ve identified the bug, here is the issue with the comment about why it’s happening: "Updated at" time incorrect in list of projects (#432410) · Issues · GitLab.org / GitLab · GitLab
Longer story: The “Updated” time is based on 3 different sources from our backend: “last_activity_at”, “last_repository_updated_at”, and “updated_at”. Whichever one of these is most recent will be displayed. The problem however is that the “updated_at” source is not something that should be used for this, as it also includes activities that don’t count as activity or updates, and is also related to our caching.
This did not stand out though for a long time, because of another bug causing this value not to be used for the actual caching. This was updated recently (Project's last updated time not accurately reflected in Projects Dashboard (#18279) · Issues · GitLab.org / GitLab · GitLab), and now that this works as expected, the other bug (that this is not the right data to use in the UI) is suddenly standing out way more.
As mentioned above, we have an issue open for that and will hopefully get to it soon, but that team is currently working on other high priority initiatives, so if anyone wants to contribute to get this done faster, there is a lot of guidance already in the issue about how this could potentially be fixed and you could ping the engineers involved in the issue for advice or the MR review.
I hope this at least explains why this is happening, even if I can’t offer a direct solution. Thanks everyone for reporting this! 