Until you added the CASH position to the portfolio, the app treated your purchase (and sale/dividend) transactions as funded by an external source. Once you add the CASH position, the app switches to treating the CASH position as the funding source for all purchases (and the destination for sale/dividend proceeds), and it does so retrospectively. This is why you see the negative value of you CASH position immediately after adding the CASH position to the portfolio.
You can address this in two ways. Simple but less precise - enter a Balance CASH transaction and indicate how much cash you have in the account now. Or, and this is especially desirable if cash in your brokerage account generates interest, go back to when you opened the account and enter all funding, withdrawal, and interest transactions. The app will include the interest in the total return calculations.