I can think of 2 possibilities:

I'm pretty sure somewhere on this forum are reports that not all U inventory is the same. This fare bucket is used for both awards and upgrades and U can be released for awards but not upgrades and vice versa. In practice, I've never noticed this - if I see U = 1, I've been able to book either an award or an upgrade.

We've also seen reports of agents not seeing elite only U availability. That doesn't explain why you saw it both for a bookable award on AY's website and on expertflyer.

If it were me, I'd call in now and check again, and if you get told it's not for you, wait until Finland CS opens for the day and call again.