Sorted the issue, basically was me with bad indentation and mispelled the <RelationshipName> for the incident 1:N relation
Ref: https://docs.microsoft.com/en-us/power-apps/maker/model-driven-apps/add-documents-tab-entity-main-form#xml-sample-for-adding-the-documents-tab-to-a-form
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<control id="DocumentSubGrid" classid="{E7A81278-8635-4d9e-8D4D-59480B391C5B}" indicationOfSubgrid="true" uniqueid="{Make an unique GUID}">
<parameters>
<ViewId>{0016F9F3-41CC-4276-9D11-04308D15858D}</ViewId>
<IsUserView>false</IsUserView>
<RelationshipName>Incident_SharepointDocuments</RelationshipName>
<TargetEntityType>sharepointdocument</TargetEntityType>
<AutoExpand>Fixed</AutoExpand>
<EnableQuickFind>false</EnableQuickFind>
<EnableViewPicker>true</EnableViewPicker>
<ViewIds />
<EnableJumpBar>false</EnableJumpBar>
<ChartGridMode>Grid</ChartGridMode>
<VisualizationId />
<IsUserChart>false</IsUserChart>
<EnableChartPicker>false</EnableChartPicker>
<RecordsPerPage>10</RecordsPerPage>
<HeaderColorCode>#F3F3F3</HeaderColorCode>
</parameters>
</control>
