This is very common.

I flew to FRA on Dec. 28. Exact number changed when I looked in the time closer to flight, but I there were 60 - 70 E+ seats empty at T-24 (+ maye 4 or 5 middles in E-). Don't know for sure the flight went out 100% full, but at most only a handful of middles went empty.

I came back on 180 earlier this week (CO 772), and even with this config with a lower amount of E+ seats, I think I counted ~45 E+ seats empty at T-24, and nothing in regular economy. There were I think two seats empty on the entire flight.

To the OP: it's certainly possible that many of those seats will stay open - it does happen (my wife got what she calls an "economy lie flat" on EWR - BOM once - though it was New Years Eve or maybe Dec. 30 a couple of years ago). Though strong likelihood that most if not all will fill up. And this is why we can't trust the seat map.