A Joint Account will report on the total funds within that account, held (with equal responsibility) by the two account holders. So spending by either party will also show as an adjustment to this total. This is how it acts now.
But Monzo does know WHO in the JA did a particular spend and so adding an extra layer of granularity reporting i.e. allowing the additional breakdown of what YOU (‘Gamma’) has spent today along with the overall JA spending (‘Gamma’ + ‘Delta’) should be possible. It just isn’t implemented.
I’m sure this has been requested a few times in the past. I’ll have a search…
Quick search over. Here are a couple of similar requests;