Although I'm still suspicious about the booking code, that's probably a red herring at the moment.

I suspect the more likely explanation is space. There are no seats in Economy left for sale tomorrow. The flight is full. Chances are they can't check you in to the seat you paid for due to this. You may or may not get moved if you ask politely at the airport. There are business seats so they might upgrade you I suppose. Not much point making a big stand though, since the hands of the check in staff might be tied.

I'd suggest firing in a complaint however if things don't work out. When you do, worth pointing out the premium you paid for the ticket if a cheaper seat was available. However, if the Economy Plus price was the only seat price left, then technically all they owe you is a free meal and the right to change your flight - both of which you could probably still get. I'm sure you will get the correct miles eventually.