I know others have reported similar things happening from time to time, but this has happened with my local bank 2x in recent weeks. Both times a simple "Reset Connection" has fixed it. The first time 2 weeks ago it was the Savings Account balance that got messed up. Today it was the checking account. It doesn't mess up (so far) in classic as I can see the downloaded balance is correct there. I update both around the same time each morning.
Here's what it does. Say the balance is $5000 with a $100 pending. Simplifi downloads and says the balance is -$100 with a pending of -$5000. It reverses the pending with the cleared! In both cases, the calculated balance is correct at $4900 without the erroneous pending.
I think it would probably correct with the next download though if I waited.