Why was my share added to next month’s rent and not this month’s?
A share is only ever added to a charge nobody has started collecting. If this month’s rent had already been presented for payment, the share goes on the next one.
From the guide Utility Billing: Splitting a Building’s Utility Bill Between Its Units.
Where the share is charged
A share is added to the tenant’s earliest future rent charge that nobody has started collecting yet. If the rent has not been generated that far ahead, the share waits and is added as soon as it is. If the lease terms change and the future charges are rebuilt, the shares move across to the rebuilt charges. If the lease ends before its next charge, the share becomes the owner’s. On a lease where roommates split the rent, the share is divided between them in the same proportions as the rent.
Read the whole guide: Utility Billing: Splitting a Building’s Utility Bill Between Its Units