Hi there.
I have the exact same problem. When I start the Lab and the Visual Studio Code loads up, I can quickly see the Terminal showing up but then it simply disappears. Same happens if I open a new Terminal. Regarding testing with other browsers; it mentions in the Lab Troubleshooting Guide that Google Chrome is the recommended browser to use.
However; I found a solution for this problem.
In the attached screenshot; you can see a circle with an x inside (next to explorer). If you hover over it; it should say No problems. Click on it and it will bring up the Terminal again.
Hope this helps.