On the mobile app, in the Budget tab, the currency symbol should be based on the primary currency of the budgeting accounts, not the 'Default Currency' in the app.
Currently, the currency symbol for Overview totals, Activity, and Budgetable/Available totals changes based on the 'Default Currency' setting. Only the currency symbol is affected, not the values themselves.
Also, in the Inflow and Outflow tables, if you don't have a 'Default Currency' selected, then it displays the currency symbol of the 'Budgeted' amount in the 'Activity' and 'Available' columns, even though the amount value displayed in those columns is in another currency (the Primary Currency).