Hello Peter,
It seems that the issue is from WS Form since I'm using it to upload the files.
Message from Bricks team:
"The root cause of your issue is the upload form.
The form changed the way to store the file into non-array value.
(And that's the reason why you use the official Metabox Theme code isn't work)
[...]
In this example, I uploaded the files via the wp backend, with no error and you can see the var_dump is standard like Metabox Theme code, the file Id stored in an array of the field name key.
[...]
This post uploaded files via the form, triggering the fatal error.
"
I'll ask them about it, thank you for your reply!