It seems that views can work in the Open data portal if created in the right way. I've not tested this fully yet and I can't remember how I created the view that doesn't work but this could be the answer for now. Here is a response from ESRI UK who also looked into this for us
Method 1 to create a view layer results in the whole layer (not the view only) being exposed in the Open Data site
Under the Item details of the parent layer, select ‘Create View Layer’
From the item details page of the view layer, select ‘Open in Map Viewer’
On the Contents Pane, click the filter icon to set up a filtered view of the layer. Click ‘Apply Filter’
From the layer in the Contents Pane, click the three dots for ‘More Options’ and click on ‘Save Layer’
When viewed on the Open Data site the entire dataset (not the filtered view) is visible.
Method 2 to create a view layer results in just the view layer (filtered dataset) being exposed in the Open Data Site.
Under the Item Details of the parent layer, select ‘Create View Layer’
From the item details page of the view layer, select the ‘Visualisation’ tab
Click the filter icon to set up a filtered view of the layer. Click ‘Apply Filter’
Select ‘Save Layer’ at the top of the page
When viewed on the Open Data site, only the filtered view is displayed es expected.