Thank you for contacting us about this.
To assist you on the issue, I would need to know which AppBuilder Client are you using - Desktop
Additionally, we are aware that in the AppBuilder Desktop Client, when you use backtick for a multi-line code, the code editor marks it in red. However, the application is successfully built and run afterwards. This said, have you tried that
and if so, are you experiencing issue with it or it is just the editor that fails on your side?
I am looking forward to you reply.