The Mac just goes to the Catalina drive and refuses to deal with booting from the external SSD (with Mojave) plugged directly into the USB port.What are the symptoms (i.e. what do you see when you try to boot macOS Mojave from the external drive)?
Of course, when I try the same thing on my 2012 Mac Pro, the Mac Pro just shuts down, because it does not support Catalina.
I can boot from an external SSD that has Catalina on it, but not Mojave. So, I'm envious and wondering how people are booting into Mojave with a Catalina-updated MacBook Pro.