My housemate and I have a shared tab. We put billls but also ad hoc things like activities and meals out on there too. Periodically we will settle the tab, resulting in one payment amounting to the net balance. As a single payment, this does not deduct spending from the categories of the payments on the tab, in the same way ‘splitting’ a payment would. This renders budgeting on Monzo impossible if you use shared tabs heavily.
Ideally, adding a payment to a shared tab should function the same as a ‘split’ with the amount you are owed on each payment showing as ‘pending’ until the tab is settled. Settling the tab then ‘approves’ all of the split payments at once. This may result in multiple payments in the back end, but you could obscure this to the user on the front end.