[CentOS] tmp option of crypttab

I have successfully used the swap option of crypttab (# man crypttab)
to encrypt the swap partition dynamically. rc.sysinit enables that 
swap partition successfully at the right point (after encryption). 

The same doesn't work for the tmp option of crypttab (# man crypttab).
The encrypted partition is present after booting the system. Manually   
mounting it works but adding "/dev/mapper/luks-tmp" into fstab shows that 
the boot process tries to mount it to early (not encrypted yet). 

This is confusing because other encrypted volumes (not dynamically) 
in fstab are successfully mounted. It seems that volumes with random 
keys are skipped at that stage. 

Does someone use a tmp directory with dynamic encryption enabled?

# cat /etc/centos-release 
CentOS release 6.8 (Final)

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LF