Hi @Marc_Chriqui, did you manage to find a solution ?
I did try to replicate your Function2 > Function1 > OnClickEvent above, and as for the Backendless team, it seemed to work OK.
However, in my context, I run into the same kind of problem you have. Here is my similar issue : a TypeError [...] is not a function error message.
@Backendless, in case you can help me understand, my app id is D7075715-5086-625A-FFAB-39C2F40FB200. In my case the problem happens immediately when loading the page B2-edit-my-boat. There are two custom functions, one calling the other and this happens consistently.
Please note this also happens when a single function is called from a reusable component, somehow like here : UI Builder Custom Function Not Working in Reusable Component
In that case, I first get an error message on application load reading Uncaught SyntaxError: await is only valid in async functions and the top level bodies of modules and then the TypeError […] is not a function happens on execution.