Nested custom xtypes not calling optionsProvider

We are using a custom xtype to create cusom multifield lists. The xtype takes the declarations from our dialog, and creates repeatable sections based on your standard dialog xml.

We have created another custom xtype, which is a select box, that populates the options based on a function which returns some values in a JSON object from a content node. It works nicely using optionsProvider.

What happens though is when we declare our select xtype within our multifield xtype, the optionsProvider does not fire, hence not populating the options.

Does anyone know if there is a known issue whereby optionsProvider isn't called in such circumstances (nested xtypes)?