Yes, read only.
So where did the call to Truncate come from and the opening with Write 
Can you try compiling cmount for FreeBSD. If you run go build -tags cmount from the root of the rclone source directory then you'll make an rclone with cmount capabilites (you'll need the fuse dev package and the fuse library and a C compiler installed).
Can you try that (just use rclone cmount instead of rclone mount) and see if it has the same problems? That should help narrow things down a bit.