the arrears would likely be ftb for the drop for not having taken child support action, up until your most recent payment (so last week) - the drop in payment would be because the system has now received the information about your child support assessment and is reducing the payment for balance of financial year to counter any over payment. the system can't stop arrears being generated to offset overpayments for csa assessments and mandatory adjustments - that can only take place from the day the information is received. so the child support assessment may have had a date of, say, July last year - so the system has received that information this week and recognised that you were underpaid for no child support action from july to may and paid the arrears BUT, the child support assessment meant your payment should have been reduced by part of that amount that you were paid in arrears, and is now reducing the rest of the payments for this financial year to counter that over payment. unfortunately it's a limitation of the system. i have discussed this exact scenario at length with people higher up than me so that it can be addressed. none of us can understand why the system pays arrears and then reduces future payments for to counter that "overpayment" - but it is what it is kwim?

as to the address thing - that is probably because your ftb claim is on the system and preventing updates from occurring. it would be noted on the system that it needs to be done though.