in reply to -s test option returns differently in some cases
I wonder if you're suffering from NFS attribute caching?
My guess is that it could be reading the local stale cache and has just not seen the update yet. If you sleep for more than the attribute cache timeout (set in the NFS mount) it should correct itself. You could try a loop that sleeps for one second and then tests the file and see if it ever recognises the change.
Also, have you closed that file that you're writing to? I don't remember if this will flush the cache or not & I don't have a NFS server set up it test it on.
Have you checked that the nfs mount options are the same on both machines & that the defaults didn't change between versions?
You can set noac on the nfs mount but it does slow things down. Sadly, NFS is showing it's age and doesn't make a very good cluster file system.