Hi Deon -
Firstly I want to apologies for such long identification of your problem and any inconvenience that caused.
The mapping approach that has been used on your page has incapsulated order of invocation: 1st 'qry_Property_Accom' on success invokes - 2nd 'qry_Accom' (which will be cloned for your final service) - 3rd 'qry_Accom_Gallery' services order.
this leads to the following:
1). Mapping from element generated (cloned) by mapping earlier (qry_Accom_Gallery datasource - Before send mapping). Appery.io doesn't support such mapping order in the core.
2). Mapping to element generated (cloned) by mapping earlier (qry_Accom_Gallery datasource - Success mapping). Does not supported also.
What does it means by not supported here, is that the initial mapping is waiting for input elements from the previous mapping structure which are marked as cloned elements, but the invoked mapping don't receives them as they are initially empty for it. Yes, we can say that this is some sort of limitation now in our core mapping structure in Appery.io.
The above said is also leads to Expand event for collapsible block that has no input data and which leads to a feature of this element event that in iterated manner looks for input data that are empty and again initialize the entry point for this event.
The general approach for Collapsible set here is to better use all such logic within one service call, so that all data will be retrieved in one request by using for example pointers to retrieve your collections data.