I'm using the Simple Tracking Method with Vanguard Brokerage Accounts. Downloading positions for Vanguard sets the cash position on each of my brokerage accounts to 0. For accounts with a non-zero cash balance, I update the cash balance using the Update cash balance action. This works well enough for positive cash balances.
When I enter a negative cash balance, it appears to accept the negative balance, but does not apply it to the account, i.e. the account total is unchanged. It apparently stores the negative balance since when I attempt to reenter it, it replies "No adjustment required.", but no cash balance is listed on the account, and the negative balance is not reflected int the total.