DAX Calculation doesn't work when collapsing groups

I have a composite model that is connected to (3) remote models (shared datasets). 

As part of one report, the Rows on the report matrix need to have a custom sorting based on a 'Wamp Name' column, so we created a Custom sort local table to handle that.

Each Wamp contains a 'Brand Group' and in the report heirarchy the brand groups are grouped under each 'Wamp Name'. Hence the report appears like the screenshot below. The last 'NRperHLDeltaYrlyAvg' measure column is the difference between the 1st and 2nd measure columns.

The Brand Table has a relationship to the local sort order table based on the 'Wamp Name' so that the sorting is based on the Wamp Order from the local table. 

Now, this all works fine when all the 'Wamp Name' groups are expanded on the group. The problem occurs when one of these groups is collapsed.

When all groups are expanded:

'NRperHLDeltatoYrlyAvg' calculation works correct

When one of the groups is collapsed (here Premium group is collapsed):

'NRperHLDeltatoYrlyAvg' calculation doesn't work. All values are 0.

The interesting part is that the first 2 base measures seem to be showing up the correct values when expanded or collapsed but it doesn't work that way for the 3rd measure calculated from these 2. We have tried creating different calculations to get around this on the local measures table but nothing seems to work when a level on the 'Wamp Name' is collapsed.

Can anyone advise a way to get around this? Any ideas appreciated!