Yes I just ran into it. Seems like it was mentioned a couple of times, but with no meaningful replies:
https://community.monzo.com/t/account-balance-on-transactions-endpoint/79461
https://community.monzo.com/t/getting-balance-after-each-transaction-through-the-api/28959
I was hoping not to have to mess around keeping a running tally of the balance as a workaround… Cheers anyway!