i had issues with setting the immutable flag on a file that gets copied into a snapshot, it rendered all snapshots that contained that file unable to be deleted normally and filled my ssd up.
i couldn’t even change the file in the snapshot because it had inherited its status from the original, so i had to remove the flag on the original and then go and manually remove that flag on all the snapshot copies before removing the old snapshots in their entirety.
here’s a link to my adventures with the immutable flag: