xtrace comparison:

# original
[root@localhost-live bug_airmon-ng-email]# set -x; for i in $(ls /sys/class/ieee80211/); do if [ ! -d "/sys/class/ieee80211/${i}/device/net" ]; then echo "$MISSING_INTERFACE" :: "$i"; fi; done; set -
++ ls --color=auto /sys/class/ieee80211/
+ for i in $(ls /sys/class/ieee80211/)
+ '[' '!' -d /sys/class/ieee80211/phy0/device/net ']'
+ set -

# change added
[root@localhost-live bug_airmon-ng-email]# set -x; for i in /sys/class/ieee80211/*; do if [ -e "${i}" ] && [ ! -d "${i}/device/net" ]; then MISSING_INTERFACE="${i}"; echo "$MISSING_INTERFACE" :: "$i"; fi; done; set -
+ for i in /sys/class/ieee80211/*
+ '[' -e /sys/class/ieee80211/phy0 ']'
+ '[' '!' -d /sys/class/ieee80211/phy0/device/net ']'
+ set -

Comments had read...

    #this should be fixed, but it's not going to be right now
    # shellcheck disable=2045

...which referred to...

https://www.shellcheck.net/wiki/SC2045

Fix setting MISSING_INTERFACE to the expected value, from commit e833818.