What
Directory sync can work from an attached clone, a copy of a repository linked in from elsewhere. For these sources it now treats the other machine as keeping the files, and before syncing it checks a switch: unless that switch reads "on", sync reports that it is switched off. Shutdown now also records whether it was interrupted in the middle of sending changes.
Why
In practice, syncing an attached clone now depends on that switch being on, so it may do nothing until the switch is turned on.
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What controls the switch, and whether it is a remote setting, is not known.