Hello!
Balance sheet accounts will show the same values between YTD and MTD (or V#Periodic with time set to a month) because balance sheet accounts are by definition a balance as of a point in time.
Periodic view in OS is dynamic based on your time selection but it will really only matter for income statement accounts which are balances over a period of time. Hopefully that helps.
If you're looking for balance sheet accounts across a specific time period, you'll have to run a calc taking the balance at the end of the period minus the previous period to see the change in the balance. We have this built into the Flow dimension as calcs but maybe you have it elsewhere or would rather do it manually via a formula in the QV.