https://openstagecontrol.ammd.net/docs/widgets/properties-reference/#matrix_props:
Advanced syntax blocks (@{}, OSC{}, JS{{}} and #{}) are resolved at the matrix’ scope (ie @{this.variables} returns the matrix’ variables property, not its parent’s)
If you want to have scoped advanced syntaxes you’ll need to use a matrix of clone widgets instead (see clones_and_variables.json example from this thread).
I'm considering adding a way to pass these syntaxes to the matrix' children without interpreting them to make this kind of situations easier.