ACr has two layouts of A321, one where the missing seat is 17A/F, one where the missing seat is 16A/F (or 18A/F, I forget - it is off by one). Or so I was told after the following incident:

I almost got absolutely burned on this where I knew I was sitting in the window seat with no seat in front of me, but they had changed planes - still an A321, but the exit row numbering not the same. So I boarded, the GA didn't flag anything, I didn't check the seat number and just sat in what I thought was the right seat, with the empty space in front of me. We were almost boarding complete when someone boarded and said I was in their seat and I realized I had a BP for a non existent seat, exactly the same situation as you.

I flagged down a FA and talked to the in-charge who told me the only economy seat was something like 30B and I could move there and I would be flying YYZ - SAN like that. I told her that wasn't happening, so she got the GA down there and when I explained the whole thing and that I was not impressed going from the best economy seat on the plane to the worst because of their screw-up the GA caved and stuck me in Premium Rouge for the flight instead.

So either what the GA told me was true (there are A321 fins with a slightly different layout) or their IT screws this up with some regularity.