Flying ANA on a UA codeshare tomorrow.

At booking (on ua.com) the seat map for this flight showed a mostly empty plane, and let me select a seat towards the front. However this selection did NOT stick.

After booking I was not able to select a seat on ua.com. However I was able to select a seat on ana.co.jp using either the United or ANA confirmation numbers. However the seat map shown was around 80% full, and the only non-middle seats available were towards the back of the plane. The seat I selected on UA during booking was shown as not available, which is possibly why my selecting it didn't work. I selected a seat towards the back, and it stuck, and was also immediately shown as being my seat on both ANA and UA's websites.

Today at check-in, the plane is mostly empty, and I was able to select a window seat near the front. I'm guessing the difference between the seat maps that I saw on ua.com during booking and ANA's website were seats that were blocked by ANA?

However one way or another, no issues selecting the seat on ANA's website, even for a UA codeshare.