Thanks for your help everybody. The reason I posted without much prior research was that I was about to board a flight and had planned to read the documents in question on the plane. That's obviously my fault for not checking the sync had worked earlier, but Syncthing had been so reliable before that it never occurred to me anything might go wrong.
Three days later, 5 of the 6 files have synced correctly. One though is still only existant as a .tmp file. Is there anything I can do short of deleting the file from the folder and then putting it in there again? I'm flying again tomorrow and this time I definitely want to have the file with me. What would happen if I deleted the .tmp file?