running shell script starts new shell instead of running the shell script - bash

Question

I don't know what to title this. Title suggestions welcome.
I'm using zsh as my default shell for the user I'm using. Whenever I try and run a script that uses bash it starts a new login shell of zsh, rather than running the script. If I change the #!/bin/bash line to #!/usr/bin/zsh it just works. Here's the kicker. Once that new shell is started the script starts working fine from that point on.
I'm using mate-terminal with vcxsrv as the X server. I initially start up a plain ubuntu bash terminal, which I then use to spawn mate-terminal, as I need the DISPLAY var setup and what not before starting mate-terminal. Anyhow, here's the script I'm attempting to start.
It's also important to note that if I reverse the shells by making bash default, and using zsh inside the script, the same thing does not happen.
How do I make this just run the script?
$ cat tmp.sh
#!/bin/bash -x
echo hello
$ ./tmp.sh
+ source /usr/sbin/start-systemd-namespace
++ SYSTEMD_EXE='/lib/systemd/systemd --system-unit=basic.target'
+++ ps -eo pid=,args=
+++ awk '$2" "$3=="/lib/systemd/systemd --system-unit=basic.target" {print $1}'
++ SYSTEMD_PID=562
++ '[' myuser '!=' root ']'
++ '[' -z 562 ']'
++ '[' 562 '!=' 1 ']'
++ export
++ sed -e 's/^declare -x //;/^IFS=".*[^"]$/{N;s/\n//}'
++ grep -E -v '^(BASH|BASH_ENV|DIRSTACK|EUID|GROUPS|HOME|HOSTNAME|IFS|LANG|LOGNAME|MACHTYPE|MAIL|NAME|OLDPWD|OPTERR|OSTYPE|PATH|PIPESTATUS|POSIXLY_CORRECT|PPID|PS1|PS4|SHELL|SHELLOPTS|SHLVL|SYSTEMD_PID|UID|USER|_)(=|$)'
++ export 'PRE_NAMESPACE_PATH=/usr/local/opt/coreutils/libexec/gnubin:/usr/local/bin:/home/myuser/bin:/home/myuser/.rbenv/bin:/usr/local/opt/coreutils/libexec/gnubin:/usr/local/bin:/home/myuser/bin:/home/myuser/.nvm/versions/node/v14.12.0/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/mnt/c/Program Files/WindowsApps/CanonicalGroupLimited.Ubuntu20.04onWindows_2004.2020.812.0_x64__79rhkp1fndgsc:/mnt/c/Program Files (x86)/Common Files/Oracle/Java/javapath:/mnt/c/WINDOWS/system32:/mnt/c/WINDOWS:/mnt/c/WINDOWS/System32/Wbem:/mnt/c/WINDOWS/System32/WindowsPowerShell/v1.0/:/mnt/c/WINDOWS/System32/OpenSSH/:/mnt/c/Android:/mnt/c/Windows/System32:/mnt/c/Program Files/Docker/Docker/resources/bin:/mnt/c/ProgramData/DockerDesktop/version-bin:/mnt/c/Users/trent/AppData/Local/Microsoft/WindowsApps:/home/myuser/go/bin:/home/myuser/.local/bin:/home/myuser/go/bin:/home/myuser/.local/bin'
++ PRE_NAMESPACE_PATH='/usr/local/opt/coreutils/libexec/gnubin:/usr/local/bin:/home/myuser/bin:/home/myuser/.rbenv/bin:/usr/local/opt/coreutils/libexec/gnubin:/usr/local/bin:/home/myuser/bin:/home/myuser/.nvm/versions/node/v14.12.0/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/mnt/c/Program Files/WindowsApps/CanonicalGroupLimited.Ubuntu20.04onWindows_2004.2020.812.0_x64__79rhkp1fndgsc:/mnt/c/Program Files (x86)/Common Files/Oracle/Java/javapath:/mnt/c/WINDOWS/system32:/mnt/c/WINDOWS:/mnt/c/WINDOWS/System32/Wbem:/mnt/c/WINDOWS/System32/WindowsPowerShell/v1.0/:/mnt/c/WINDOWS/System32/OpenSSH/:/mnt/c/Android:/mnt/c/Windows/System32:/mnt/c/Program Files/Docker/Docker/resources/bin:/mnt/c/ProgramData/DockerDesktop/version-bin:/mnt/c/Users/trent/AppData/Local/Microsoft/WindowsApps:/home/myuser/go/bin:/home/myuser/.local/bin:/home/myuser/go/bin:/home/myuser/.local/bin'
+++ pwd
++ export PRE_NAMESPACE_PWD=/home/myuser
++ PRE_NAMESPACE_PWD=/home/myuser
++ exec sudo /usr/sbin/enter-systemd-namespace ''
Welcome to Ubuntu 20.04.1 LTS (GNU/Linux 4.19.128-microsoft-standard x86_64)
* Documentation: https://help.ubuntu.com
* Management: https://landscape.canonical.com
* Support: https://ubuntu.com/advantage
System information as of Mon Oct 12 21:48:02 MDT 2020
System load: 0.0 Processes: 66
Usage of /: 7.2% of 250.98GB Users logged in: 0
Memory usage: 7% IPv4 address for eth0: 172.26.153.138
Swap usage: 0%
* Kubernetes 1.19 is out! Get it in one command with:
sudo snap install microk8s --channel=1.19 --classic
https://microk8s.io/ has docs and details.
21 updates can be installed immediately.
0 of these updates are security updates.
To see these additional updates run: apt list --upgradable
Last login: Mon Oct 12 21:42:37 MDT 2020 on pts/3
* keychain 2.8.5 ~ http://www.funtoo.org
* Found existing ssh-agent: 108201
* Known ssh key: /home/myuser/.ssh/id_rsa

Answer 1

The key was this line in /etc/bash.bashrc
source /usr/sbin/start-systemd-namespace
Awhile back I got systemd working from some instructions elsewhere on the net. It must have added this line. It causes an entirely new shell to start when you're using another shell like zsh. After commenting it out, voila, no more problem.