Many thanks for the quick response. Yes I think it is the non-native NTFS driver. I use it because I multi-boot Windows and Linux on this PC and I sometimes want to use Windows on the same files. So far I’ve got away with it. It’s a pity there isn’t a filesystem which is officially supported by both OS’s.
So I’ve moved the Syncthing folders to an Ext4 partition and all is well. I can install Syncthing on the multibooted Windows instance although this means duplicating the files. As it happens I have enough space so it’s not a big problem.
Incidentally I had to do this with Dropbox (which I’m trying to replace) as I used to use NTFS for that, but then a couple of years ago they refused to work with NTFS on Linux so I had to duplicate the Dropbox files on an Ext4 partition. The daily changes aren’t big so I can work with it.
I have another slight issue which I will put on a different post, but this issue is resolved albeit not as I would have ideally preferred.
The takeaway for anyone reading this thread is that using the ntfs driver on Linux causes conflicts.