/mnt permissions keep changing on EC2 instances

/mnt permissions keep changing on EC2 instances

I have a few Ubuntu instances on EC2 where I use the /mnt instance storage
to keep logs and other fungible stuff. In order to store user logs the
/mnt directory needs to have 777 permissions:
$ sudo chmod 777 /mnt
But periodically (about daily) permissions revert back to 755, for no
apparent reason. Is there any way to make /mnt have write permissions for
all users -- and keep it that way? I already have a task that runs on
startup with the above line, but apparently it does not work.
A periodic task would be a last resort; honestly, I would like to
understand the problem and solve it correctly. I have searched here and on
the intertubes, no luck so far -- it is not an easily googleable question
anyway.
Perhaps some mount options would do the trick? My fstab:
/dev/xvdb /mnt auto defaults,nobootwait,comment=cloudconfig 0
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