How to Downgrade from MacOS Catalina to Mojave- Creating a bootable hard drive in APFS format
The Apple documentation How to create a bootable installer for macOS says to use Mac OS Extended not APFS.  

Mac OS Extended is the new name for HFS+


  Use the 'createinstallmedia' command in Terminal
  
  
  Connect the USB flash drive or other volume you're using for the
  bootable installer. Make sure that it has at least 12GB of available
  storage and is formatted as Mac OS Extended.
  Open Terminal, which is in the Utilities folder of your Applications folder.
  Type or paste one of the following commands in Terminal. These assume that the installer is still in your Applications folder, and
  MyVolume is the name of the USB flash drive or other volume you're
  using. If it has a different name, replace MyVolume in these
  commands with the name of your volume.
  
  Catalina:*
  
  sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
  
  Mojave:*
  
  sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
  


From the link in the above quote


  Are you preparing a Time Machine backup disk or bootable installer?
  
  Choose Mac OS Extended (Journaled) for any disk that you plan to use with Time Machine or as a bootable installer.


As you can see the documentation is somewhat inconsistent regarding journaling (it probably doesn't matter) however I used Mac OS Extended (Journaled) to install Mojave and it worked fine.