Yeah I honestly don’t have a good answer for this– my best guess would be that zapper updated something regarding how they calculate the value of yUSD– like maybe before they were just using the currently available swap rate on uniswap, and now they use a calculated value from the value of yCRV and amount of yCRV in the vault vs yUSD issued– but again, that’s just a guess.
However, the vault can’t lose money– like you definitely don’t have less yCRV for the amount of yUSD you had before– so I imagine it’s a calculation change on zapper’s part. Let me know if it goes down again, I’d be interested.