SynchronizeWithExternalDependencies will call Modify on the Pose Search Database when the database has changed. This will call FAsyncPoseSearchDatabasesManagement::OnObjectModified which in turn marks any dependent DDC tasks as to be cancelled. However, it seems that only FAsyncPoseSearchDatabasesManagement::OnObjectPropertyChanged actually cancels tasks that are marked to be cancelled, and the OnObjectPropertyChanged delegate is never invoked when SynchronizeWithExternalDependencies is called. So we potentially have a pending task that is never killed which could cause a soft lock.
No specific repro for this other than the discussion on the linked EPS ticket
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-307008 in the post.