Probably due to a timing issue.
This approach is OK for simple things, but when you get into complex javascript binding events and firing them you will end with quite some spaghetti code trying to make your vanilla js work in harmony with App Connect library. So I stopped recommending this approach for 3rd party libraries.
Now I recommend creating custom wappler components that leverage App Connect framework.
Here you have additional info on how to create one:
And if you are looking specifically for ApexCharts here you have a quick example I created a month ago:
It's basic but it will get you going. Since then I improved my ApexCharts custom component and I'm just waiting for the team to release custom component UI support in order to release it as an extension.
Even if you are not comfortable with JS and how App Connect works I advise you to go this route because you will get a reliable and smoother experience and less "I managed to resolve this, but I don’t know the reason."
Good luck.