Two observations I have made might be relevant, one very recent, the other longstanding.
My long-standing observation is that macOS has a large and seemingly growing class of errors, at least performance that seems in error to longtime Mac users. This class of errors is software objects do not complete updating themselves, including the user interface, as a part of processing new data. While (“traditional”) Mac users see this as a glaring bug, my wife tells me such performance is considered normal in the standard Windows experience. Years ago, Apple rapidly expanded their workforce, and also hired outside contractors as beta testers. The tight conformity which was a primary virtue of the Mac user Interface, as an aspiration as well as in the product, was lost in the process. In my long experience as a beta tester, this seems to explain many many anomalies which I consider errors.
My recent observation was of a bug which seems to be yet another specific example of this large general class of errors. I added a folder of files to a folder on my iCloud Drive that has long been shared with my wife. The new files appeared soon on her iPhone, but after a couple of hours still not on her Mac. She restarted, etc., no improvement. Then for some unrelated reason, on her Mac she added a file to that same shared folder. Apparently as a consequence, the files I had shared suddenly appeared. It seems that changing the content of the folder on iCloud Drive did not trigger an update event which would sync iCloud Drive in the cloud with iCloud drive on her Mac. Adding files to iCloud Drive on her Mac was necessary to trigger the expected update.
You can check to see if your issue is another specific example of this general class of errors. While logged in to iCloud on your MacBook Air, edit your Contacts. If my guess is right, your editing will trigger a sync, and your missing records will soon appear.
A possibility is that the extremely low syncing observed by Will M actually was no syncing at all until some seemingly unrelated event triggered the sync process.