Unable mount subvolume with "File exists" error

[MASAKI Haruka <reasonset@inflaton.co.jp>]

When run this script

***
#!/bin/zsh
setopt RC_EXPAND_PARAM

if [[ $1 == start ]]
then 
  if iscsiadm -m node -L all && mount /mnt/worldvol && mount /MasterVol && mount ~foo/local
  ...
***

failed with
mount: /MasterVol: mount(2) system call failed: File exists.
message after last month Manjaro update.

in /etc/fstab

***
LABEL=MasterBtr                           /MasterVol     btrfs   defaults,relatime,space_cache,compress=lzo,subvol=world,noauto 0 0
LABEL=MasterBtr                           /mnt/worldvol  btrfs   defaults,relatime,space_cache,compress=lzo,noauto 0 0
/MasterVol/local                          /home/foo/local none   rbind,noauto 0 0
***

I could mount every volume by unmount /mnt/worldvol and mount each volume manually, but the script always failed with "File exists."