When object is moved to thread then there are issues with callbacks triggered via JS / EM_ASM. Attached example shows this behavior when you build it with Wasam multithreaded.

In attached code if you use the line

auto thread = new QThread{this}; 

You will see two native Callbacks in chrome browser console

if you comment above line and use 

thread = QThread::create([](){trigger();}); 
 

You will see three native call backs. Actually 4 would be expected at this point