I have always considered ‘spent today’ to total up what’s left the account. Everyone will consider it differently, but that’s how I view it.
Even if your mate sends £10 immediately after you buy something on their behalf, £40 is still leaving your account before it is then replenished - and from what I can tell, all this feature does is keep a running total of Debits from the account.
Like I said, I would expect the Budgeting feature to reflect this, but not Spent Today. On that note, it doesn’t do it on Budgeting and I think that does need sorting.
Edit: I misread your reply before (though my previous points still stand); the issue with Bill splits is that for you the other party may pay back in minutes whereas someone else may have a less reliable payer causing the split to span multiple days.
Then there is the argument that despite the fact the bill was settled/split, the full transaction was initially debited from your account so is technically still your spend despite splitting the bill. I honestly think that what you truly want is budgeting to work properly, because that is where I would expect the balance to update based on credits as well as debits.