Actually, I think my issue might be partially my fault, because I was confused by a different contract bug. Initially, I had the first transfer window turned off. This meant that I had no option to negotiate new contracts with my players who were asking to extend (which is obviously not how it should be, even if transfers are disabled). Then I started a new career with the first window turned on, tried to negotiate with a player on an expiring contract, and as soon as I agreed new terms with them, saw the unchanged "11 months" remaining, and immediately quit the season, thinking it was still not working. However, I just went back to this season to experiment a bit more, and after advancing a few days in the calendar, I received a notification that I could "Finalize contract" for the player I agreed terms with. Since this is new in FC 27, I was not expecting to have to take this delayed extra step to agree contract terms, so I just bailed before it went through. When I finalized the contract, the contract length did update.
This might not be what you're experiencing—I know you mentioned you "did the process of finalising"—but I figured I'd lay that out just in case.