@frank the tank This isn't specific to TOS, I've used IB for years and see the exact same thing. The LAST is almost always the MID and I know that isn't correct. The MID is calculated from the individual legs, so that number can be pretty far off also when trying to get filled. The PnL showing on the individual trades is not even close most days.
It's frustrating but just shows how much work you need to put in sometimes to get better results. For example, when I get an instant fill, I know I've probably left a few bucks on the table.