When ref sync (fork syncing) applies upstream changes to forks, if any refs are updated and then quickly deleted the fetch can fail because the ref no longer exists. The ref sync code includes handling for this case, to log a clear message without spamming the logs with unnecessary stack traces. Git 2.21+ include textual changes which break that handling, however.
Functionality is not impacted by this issue. Ref sync produces identical results with Git 2.21+ to Git 2.20 and earlier. The difference is that update-then-delete scenarios on Git 2.21+ will produce more verbose logging.
- Git 2.21
No workaround is available or necessary. There is no functional impact from this issue; it's purely cosmetic (logging).