Append-only mode with only sftp server (no rclone)

First of all, a big thank you to all the devs and contributors for this pretty amazing piece of software!

For various reasons, I would like to use restic in append-only mode (in case client gets compromised), with sftp only (no clone or restic server)

I have tried to run the sftp server with only the following commands whitelisted:

open,close,read,write,lstat,fstat,opendir,readdir,mkdir,realpath,stat,fsync,fsetstat,rename

And then I run the restic commands with –-no-lock (I understand I do not allow the delete permission necessary to delete the lock file).

I noticed the following:

restic snapshosts –-no-lock works with no error message

restic backup –-no-lock works, but unexpectedly complains about being unable to delete the lock file, with the following message:

processed 1 files, 0 B in 0:04

snapshot eaeff1c3 saved

Remove(<lock/53b3c4dea4>) failed: Remove /home/repo/locks/ea4b7162b7a2a139a2eca867c166f53b701: remove /home/repo/locks/ea4b7162b7a2a139a2eca867c166f53b701: permission denied

restic restore --no-lock works with no error message

On the side of the sftp server, for these 3 commands, I see a bunch of error messages, all explaining that restic is trying to use some non-whitelisted instructions.

So if I ignore the error message, it seems to all work fine.

Can someone explain to me why backup --no-lock still complain about not being able to delete the lock file?

Also, and most importantly, is this setup ok to use, or am I shooting myself in the foot in a subtle way?

Thanks!